■^ 


BIOLOGY 
LIBRARY 

G 


THE  ELEMENTARY  PRINCIPLES 
OF  ^GENERAL  BIOLOGY* 


BY 

JAMES   FRANCIS   ABBOTT 

PROFESSOR    OF    ZOOLOGY   IN  WASHINGTON    UNIVERSITY 


'Ntta  gork 

THE   MACMILLAN   COMPANY 

1914 

Ml  rights  reserved 


.0? 


^3 

BIOLOGY 

LIBRARY 

G 


Copyright,  1914, 
By  the  MACMILLAN  COMPANY. 

Set  up  and  electrotyped.     Published  January,  1914. 


TiToriaooti  i^resa 

J.  S.  Cushing  Co.  —  Berwick  &  Smith  Co. 

Norwood,  Mass.,  U.S.A. 


"  Before  the  great  problems  [of  Biology],  the  cleft  between  Zoology 
and  Botany  fades  away,  for  the  same  problems  are  common  to  the  twin 
sciences.  When  the  zoologist  becomes  a  student  not  of  the  dead  but  of 
the  living,  of  the  vital  processes  of  the  cell  rather  than  of  the  dry  bones 
of  the  body,  he  becomes  once  more  a  physiologist  and  the  gulf  between 
these  two  disciplines  disappears.  When  he  becomes  a  physiologist,  he 
becomes,  ipso  facto,  a  student  of  chemistry  and  physics." 

D'Arcy  Thompson,  —  "  Magnalia  Naturae." 


284558 


Digitized  by  the  Internet  Archive 

in  2007  with  funding  from 

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http://www.archive.org/details/abbottbiologyOOabborich 


PREFACE 

In  this  book  I  have  endeavored  to  present  in  an 
elementary  way  some  of  the  fundamental  generali- 
zations that  are  the  product  of  modern  research  in 
biology.  The  artificial  division  between  the  study 
of  plants  and  that  of  animals  is  one  that  is  becom- 
ing increasingly  difiicult  to  maintain,  inasmuch  as 
some  biological  principles  are  best  illustrated  by 
phenomena  in  the  plant  world,  others  by  those  of 
the  animal  world.  I  have  tried,  therefore,  to  utilize 
both  aspects  of  the  subject  and  to  draw  my  illustra- 
tive material  impartially  from  both  kingdoms. 

The  practice  that  insists  upon  the  student  getting 
his  knowledge  of  natural  science  at  first  hand  needs 
nowadays  no  justification.  The  laboratory  method 
of  study  has  shown  itself  to  be  not  only  the  best 
means  of  acquiring  a  concrete  and  accurate  knowl- 
edge of  the  science  studied  but  also  a  primary  pre- 
requisite for  those  habits  of  thought  that  are  essential 
to  what  has  come  to  be  known  as  the  "  scientific 
method."  Nevertheless  in  Biology  the  field  is  so 
broad  and  so  varied  that  the  student  is  very  likely 
to  lose  sight  of  the  fundamental  principles  that 
underlie  all  living  nature.  Moreover,  these  princi- 
ples do  not  grow  out  of   the   laboratory  work  so 

obviously  nor  are  they  so  easily  demonstrated  by 

vii 


viii  PREFACE 

experiment  as  is  the  case  with  such  sciences  as 
chemistry  and  physics.  This  book  is  accordingly 
planned  to  supply  a  background  for  a  laboratory 
course  in  Biology  and  to  supplement  the  facts  ac- 
quired in  such  a  course,  the  exact  nature  of  which 
will  depend  upon  the  convictions  or  preliminary 
training  of  the  individual  instructor. 

On  the  other  hand,  it  is  believed  that  the  general 
reader  also  will  find  here  a  simple  statement  of  the 
fundamentals  of  General  Biology,  a  subject  that  is 
becoming  increasingly  important  in  our  everyday  life. 

In  covering  so  much  ground  I  have  been  compelled 
to  condense  many  subjects  to  paragraphs  that  might 
well  have  deserved  whole  chapters  to  themselves. 
The  wide-awake  teacher,  I  think,  will  have  no  diffi- 
culty in  amplifying  those  portions  that  he  esteems 
most  important  or  in  which  he  is  most  interested. 
I  am  conscious,  too,  of  the  fact  that  many  generali- 
zations have  been  stated  in  a  much  less  cautious 
way  than  would  have  been  the  case  if  condensation 
had  not  seemed  so  essential  a  feature.  But,  apart 
from  this,  I  think  that  it  is  preferable,  pedagogically, 
that  a  student  should  get  a  few  clean-cut  funda- 
mental ideas  which  perhaps  require  subsequent  qual- 
ification than  that  he  should  have  vague  notions  in 
which  exceptions  to  rules  figure  as  largely  as  the 
rules  themselves.  For  instance,  it  is  best  that  he 
should  acquire  the  fact  that  the  division  of  chromo- 
somes in  mitosis  is  equal  and  that  in  consequence 
the  number  of  chromosomes  in  an  individual  or  a 
species  is  constant,  leaving  any  consideration  of  the 


PREFACE  ix 

accessory  chromosome,  important  as  it  may  be,  to 
a  time  when  the  former  concept  shall  have  taken 
firm  root. 

A  chapter  on  Animal  Behavior  was  projected  but 
was  abandoned  when  it  was  found  that  its  inclusion 
would  have  increased  the  size  of  the  volume  unduly. 
For  the  same  reason  no  apology  need  be  offered  for 
the  constant  reference  by  name  without  comment 
to  the  various  groups  of  animals  and  plants.  The 
first-hand  knowledge  of  the  types  in  the  laboratory 
will  have  supplied  the  descriptive  details  for  which 
there  is  no  room  in  the  present  work,  although  text- 
figures  have  been  freely  used  to  illustrate  the  forms 
mentioned. 

In  such  a  book  as  the  present- one,  little  can  be 
claimed  for  originality  except  the  manner  of  pre- 
senting the  subject.  I  have  sought  counsel  and 
criticism  in  those  fields  in  which  my  personal  knowl- 
edge is  least  dependable,  and  I  hope  that  such  errors 
as  may  have  crept  in  will  not  be  significant  ones. 
I  am  particularly  indebted  to  Professor  George  T. 
Moore,  Director  of  the  Missouri  Botanical  Gardens, 
who  read  the  whole  book  in  manuscript,  and  to  Pro- 
fessor Walter  E.  Garrey,  who  read  the  proof  of  the 
first  four  chapters.  Acknowledgments  are  also  due 
to  the  following  for  the  use  of  cliches  or  permission 
to  copy  figures:  to  Herr  Gustav  Fischer,  Jena,  for 
permission  to  use  figures  7,  13,  34,  55,  60,  and  82; 
to  Messrs.  Henry  Holt  and  Co.,  for  the  use  of  fig- 
ures 8,  22,  49,  92,  94,  100,  106,  and  112;  to  Messrs. 
Ginn  and  Co.,  for  the  use  of  figures  31  and  103 ;  to 


X  PREFACE 

the  American  Book  Co.,  for  the  use  of  figures  2,  52, 

97,  and  109 ;  to  Messrs.  G.  P.  Putnam's  Sons,  for 
the  use  of  figure  16;  to  Messrs.  D.  Appleton  and 
Co.,  for  the  use  of  figures  19,  58,  62,  67,  70,  90,  93, 
95,  96,  and  113;  to  Messrs.  Longmans,  Green,  and 
Co.,  for  the  use  of  figure  23 ;  to  Messrs.  P.  Blakis- 
ton's  Son  and  Co.,  for  the  use  of  figure  64 ;  to  Pro- 
fessor C.  B.  Davenport  and  the  Editors  of  the 
Popular  Science  Monthly,  for  the  use  of  figure  71 ; 
to  Professor  Davenport  and  Messrs.  John  Wiley  and 
Sons,  for  the  use  of  figure  73 ;  to  the  Columbia 
University  Press,  for  the  use  of  figure  110;  to  Pro- 
fessor John  Schaffner,  for  permission  to  copy  figure 
66;  to  Professor  C.  C.  Curtis,  for  the  use  of  figures 
51  and  107;  to  the  Editors  of  the  American  Review 
of  Reviews,  for  the  use  of  figure  108 ;  and  to  Sir  E. 
Ray  Lankester,  for  permission  to  copy  figure  112. 
For  photographs  from  which  were  made  figures  85, 

98,  and  99  I  am  indebted  to  the  kindness  of  Profes- 
sor S.  M.  Coulter.  All  the  other  illustrations,  with 
the  exception  of  figures  6,  10,  24,  25,  37,  40,  42-44, 
50,  54,  5^,  57,  63,  74,  77,  88,  89,  and  114,  are  from 
publications  of  The  Macmillan  Co. 

J.  F.  A. 
January,  1914. 


CONTENTS 


CHAPTER 

I.    Living  Substance 


Living  and  Non-living    . 

Life  and  Death.     l!lemental  Death 

Chemistry  of  Protoplasm 

Proteins,  Fats,  and  Carbohydrates 
Physical  Structure  of  Protoplasm*  . 
Organization  of  Protoplasm    . 

The  Cell 


IIL 


II.    The  Primary  Functions  of  the  Organism 
Cellular  and  Non-cellular  Organization 
Functions  of  a  Free  Cell 

Locomotion,  Ingestion,  Digestion, 
ilation.  Irritability,  Reproduction 
>^  Specialization  in  Locomotor  Organs 

Specialization  in  Conducting  Organs 
Secretion         ..... 
Specialization  in  Digestion 
Summary        ..... 
Specialization  and  Differentiation 
Tissues,  Organs,  Systems 
Homology  and  Analogy  . 


Egestion,  Assim 


Metabolism         ..... 
Oxidation        ..... 
Conservation  of  Energy 
Chemical  Synthesis  in  the  Organism 

Photosynthesis 

Production  of  Fats  and  Proteins 
Dissimilation  ..... 
Metabolism  in  Animals  . 
Foods  in  General    ...» 


xii  CONTENTS 

CHAPTER  PAGE 

Fate  of  Foods  in  Higher  Animals             .        .        . '       .  61 

Role  of  Oxygen  in  Metabolism 62 

Aerobic  and  Anaerobic  Forms 63 

Combustion  and  Respiration 64 

Poisons  and  Antiseptics  .......  66 

Cycle  of  the  Elements  in  Organic  Nature        ...  68 

The  Nitrogen  Cycle 71 

Destruction  of  Organisms          .....  72 

Putrefactive  Organisms 73 

Denitrification  and  Nitrogen  Fixation       ...  75 

Nature  of  Energy  Transformed 77 

Movement 77 

Heat 79 

Electricity 79 

Light 80 

Enzymes  and  Enzymatic  Action       ....  82 

Internal  Secretions  and  Hormones   ....  86 

IV.     Growth 90 

Cumulative  Integration  .......  91 

Amitosis  and  Mitosis                         92 

Abnormal  Mitosi§     .         .         .         .         .         .         .97 

Nature  of  the  Centrosome 98 

Influence  of  External  Conditions  on  Growth  .         .         .  100 

Light  and  Heat 101 

Chemical  Agents 102 

V.     Tissue  Differentiation  for  Specific  Functions    .         .  104 

Differentiation  in  Animals       ....                  .  104 

Alimentary  System 104 

Sensory  Organs,  —  Cephalization     ....  107 

Skeletal  Structures  , 110 

Endoskeleton 110 

Exoskeleton       . 112 

Muscular  System 113 

Circulatory  System 114 

Excretory  Organs     .  .         .         .         .         .117 

Differentiation  in  Plants          ......  121 

Plant  Movement 123 


CONTENTS  xiii 

CHAPTER  PAGE 

Supporting  Structures 123 

Circulatory  System 126 

Alimentary  System . 126 

VI.    Ontogenesis 129 

Biogenesis  and  Abiogenesis 130 

Reproduction  as  a  Growth  Process  .         .         .         .131 

Fission  in  Metazoa  .         .         .         .         .         .         .132 

Fission  in  Lower  Plants    .         .         .         .         .         .135 

Temporary  Budding 136 

Permanent  Budding 139 

Spore  Formation 140 

Sexual  Reproduction 141 

Total  Conjugation 142 

Isogamy    .         .         .         .         .         .         .         .     142 

Anisogamy  .         •         .         .         .         .144 

Sexual  Differentiation 147 

Partial  Conjugation 151 

Cytoplasmic  Conjugation  (Plastogamy)     .         .     151 

Nuclear  Conjugation  (Karyogamy)   .         .         .152 

1  Nuclear  Phenomena  of  Zygosis  in  Animals       .         .     155 

Cleavage 158 

Gastrulation      .         .         .         .         .         .         .161 

Further  Differentiation 162 

Conjugation  in  Protozoa 164 

Parthenogenesis 167 

Artificial  Parthenogenesis  .         .         .         .         .169 

Alternation  of  Generations  in  Animals       ,         .     171 

Sexual  Reproduction  in  Plants  .         .         .         .174 

Liverworts  and  Mosses      ....  175 

Ferns 176 

Seed  Plants 177 

Germination  of  the  Megaspore  .         .         .179 

Germination  of  the  Microspore  .         .         .179 

Parthenogenesis  in  Plants  .         .         .         .182 

Apogamy 183 

The  Probable  Evolution  of  the  Plant  World     .         .183 

Morphogenesis  .         .         .         -         -         .         .         .185 

Regeneration 185 


xiv  CONTENTS 

CHAPTER  PAGE 

Regulation 187 

Heteromorphosis  .         .         .         .         .         .         .189 

Theories  of  Morphogenesis 190 

Preformation         .......  191 

Epigenesis 192 

"  Weismannism  "  .......  193 

Vitalism  and  Mechanism 194 

Summary 196 

VII.     Vabiation  and  Heredity 198 

Variation 198 

The  Law  of  Frequency  of  Error    ....  200 

Types  of  Variation  Curves 202 

Asymmetrical  Variation          ....  204 

Discontinuous  Variation         ....  204 

Mutations 207 

Correlated  Variability 209 

Effect  of  Life  Conditions  on  Variation  .         .         .211 

Causes  of  Variation 212 

Heredity 214 

Heredity  and  Inheritance 214 

Individual  Heredity  and  Racial  Heredity      .         .  216 

■'    Galton's  Law  of  Ancestral  Inheritance           .         .  217 

Filial  Regression 219 

Effect  of  Selection  in  Heredity      .         .         .         .  220 

Pure  Lines 221 

Unit  Characters  and  Mendelian  Inheritance          .  223 

Sex-limited  Inheritance           ....  233 

Economic  Aspects  of  the  Subject           .                  .  234 

The  Inheritance  of  Disease    ....  236 

The  Inheritance  of  Defects    ....  239 

Eugenics 240 

VIII.    Organic  Response 242 

Environment 243 

The  Usual  Conditions  of  Environment         .         .         .  244 

Temperature          .......  244 

Light 245 

Chemical  Environment          .....  245 


CONTENTS  XV 

CHAPTER  PAGE 

Nature  of  Organic  Response 246 

Electric  Response 247 

Individual  Response  to  Unsymmetrical  Stimuli       .  248 

Adaptive  Response 253 

Immunity 255 

Morphogenetic  Response 256 

Non-adaptive  Morphogenetic  Response    .         .  257 

Influence  of  Food 258 

General  Adaptation 259 

Some  Types  of  Adaptation 260 

Aquatic  Organisms   ......  260 

Aerial  Adaptations   ......  262 

Subterranean  Adaptations         ....  263 

Protective  Adaptations 266 

Protective  Coloration         ....  267 

Specific  Resemblance          ....  268 

Aggressive  Resemblance     ....  269 

Mimicry 269 

The  Care  of  the  Young      .         .         .         .272 

Environmental  Adaptations  of  Plants      .         .         .  274 

Adaptations  for  Seed  Dispersal         .         .         .  278 

Associations  of  Animals 279 

Commensalism 280 

Parasitism  in  Protozoa     ......  283 

Parasitism  in  Worms        ......  285 

Parasitism- in  Insects        ......  287 

Sacculina          .         .         .         .      •  .         .         .         .  289 

Associations  among  Plants 290 

Lichens 291 

Parasitism  in  Plants 292 

Associations  of  Plants  and  Animals          ....  293 

Grafts 294 


IX.    Species  and  Their  Origin 296 

Meaning  of  Species 296 

Polymorphism 301 

Elementary  Species  and  Linnaean  Species         .         .  305 


xvi  ,  CONTENTS 

PAGE 

The  Origin  of  Species 306 

Evidence  for  the  Evolution  of  Species  in  the  Past  .  307 

History  of  the  Elephant 308 

Vestigial  Structures 312 

"Darwinism" 314 

Lamarck's  Theory 317 

Critique  of  the  Darwinian  Theory  ....  318 

Critique  of  the  Lamarckian  Theory          .        .        .  320 

Conclusion 323 


THE  ELEMENTARY  PRINCIPLES 
OF   GENERAL   BIOLOGY 


» ;  .  .  J 


GEJN'ERAL   BIOLOGY 

CHAPTER  I 

LIVING  SUBSTANCE 

Biology,  the  "science  of  life,"  includes  in  its 
broadest  aspects  the  investigation  of  all  that  per- 
tains to  the  structure  and  functions  of  living  things. 
The  observing  and  recording  of  the  wonderful 
variety  of  Nature  will  always  have  a  fascination 
not  only  for  the  poet,  but  for  the  scientist  as  well. 
But  the  latter  is  more  especially  concerned  with 
the  meaning,  the  analysis,  or  the  explanation  of 
natural  phenomena.  Philosophy  tells  us  that 
science  can  never  hope  to  get  the  ultimate 
explanation  of  anything  which  it  observes.  All 
that  it  can  do  is  to  reduce  the  complexities  to  simpler 
expression,  to  find  the  common  denominator  for 
things  that  seem  at  first  glance  unrelated,  in  the 
same  way  that  the  mathematician  by  processes  of 
factoring  reduces  elaborate  and  complex  algebraic 
expressions  to  simple  statements  of  relation.  And, 
just  as  in  mathematics,  the  greater  the  number  of 
variables  we  have  to  deal  with,  the  more  involved  and 
difficult  becomes  our  computation,  so  in  physical 
and  biological   science  the  greater  the  number   of 


2  .      .;  :   GE-NfiRAL   BIOLOGY 

unknown  factors  there  may  be,  the  greater  becomes 
our  difficulty  in  reducing  them  to  fundamental 
principles.  This  is  why  biology  is  so  strikingly  an 
"inexact"  science  in  comparison  with  physics  or 
inorganic  chemistry.  Yet,  it  is  not  necessary  even 
for  the  physicist  or  the  chemist  to  know  what  is  the 
ultimate  nature  of  matter  or  force  or  electricity  or 
atoms  in  order  to  study  such  things  and  formulate 
general  laws  based  on  such  observation ;  nor  is  it 
necessary  for  the  biologist  to  concern  himself  with 
the  meaning  or  nature  of  life  in  order  to  find  out 
what  principles  govern  in  the  world  of  living  things. 

The  study  and  comparison  of  the  structures  of 
plants  and  animals,  of  their  methods  of  growth  and 
reproduction,  their  relation  to  each  other  and  the 
world  about  them,  has  revealed  the  fact  that  there 
is  an  underlying  unity  in  nature  that  makes  it  pos- 
sible for  us  to  sum  up  our  observations  in  general 
principles,  incompletely  understood,  of  course,  but 
more  or  less  applicable  to  all  living  things.  The 
consideration  of  these  general  principles  forms  the 
basis  for  a  General  Biology  in  the  sense  in  which  it 
will  be  taken  in  the  present  work. 

Although  we  shall  not  attempt  to  elucidate  life 
in  any  philosophical  sense,  it  is  of  interest,  notwith- 
standing, to  discover  at  the  start  just  how  much 
science  can  tell  us  of  the  nature  of  life,  or  of  living 
things  as  a  whole. 

Living  and  Non-living.  —  If  a  biologist  should 
ask  the  average  layman  whether  he  could  tell  the 


LIVING  SUBSTANCE  3 

difference  between  something  alive  and  something 
that  is  not,  he  would  hardly  be  taken  seriously. 
Yet,  if  such  a  layman  should  be  pressed  to  define 
just  what  he  meant  by  "being  alive,"  he  might  be 
hard  put.  It  might  be  assumed  that  some  charac- 
teristic chemical  compounds  are  to  be  found  in  liv- 
ing matter  which  are  absent  in  non-living  matter. 
But  thousands  of  exact  chemical  analyses  have 
been  made  of  every  sort  of  living  thing  and  no  ele- 
ment or  compound  has  ever  been  found  which  is 
essentially  different  from  what  may  exist  in  the 
non-living  world.  Long  ago  a  distinction  used  to 
be  made  between  "organic"  and  "inorganic" 
substances,  —  the  former  being  the  product  of 
living  "organisms."  But  such  a  distinction  has 
broken  down.  It  is  possible  to  synthesize  substances 
in  the  test  tube,  identical  in  chemical  composition 
with  those  formed  in  Nature's  laboratory,  —  the 
tissue  of  plant  and  animal.  Indeed,  the  ability  to 
artificially  reproduce  natural  products  in  this  way 
has  proved  of  great  value  commercially,  and  arti- 
ficially synthesized  indigo,  camphor,  etc.,  now 
supplement  in  large  measure  Nature's  meager  store 
of  such  things. 

Nor  is  it  easier  to  discover  any  unique  physical 
phenomena  in  living  things.  So  far  as  we  can 
observe,  —  and  the  more  our  observations  are 
extended,  the  more  is  the  conclusion  confirmed, 
—  living  matter  obeys  the  same  physical  laws  that 
obtain  in  the  rest  of  the  universe.  Again,  living 
things  grow:    but  so  do  crystals  and  clouds.     They 


4  GENERAL  BIOLOGY 

reproduce  themselves,  but,  as  we  shall  see  later, 
this  is  but  a  discontinuous  form  of  growth,  and  may 
be  paralleled,  perhaps,  in  other  "  inorganic  "  bodies. 

Life  and  Death. — If  we  find  it  so  difficult  to 
point  to  any  one  thing  as  the  touchstone  of  living 
matter  contrasted  with  non-living  matter,  what 
shall  we  say  of  the  difference  between  that  which  is 
alive  and  that  which  has  been,  but  is  no  longer,  —  in 
other  words  between  living  matter  and  dead  matter  ? 
A  turtle  may  justly  be  called  a  dead  turtle  if  we  cut 
off  its  head,  yet,  if  we  cut  out  the  heart  of  such  a 
decapitated  turtle  and  suspend  it  on  hooks  in  a 
moist  chamber,  wet  with  a  weak  solution  of  common 
salt,  such  a  heart  will  go  on  beating  rhythmically  for 
days.  So  long  as  it  beats  we  are  forced  to  consider 
the  substance  composing  it  as  living  matter. 

We  must  make  a  distinction,  then,  between 
general  life  and  death,  which  affects  the  whole  or- 
ganism and  elemental  life  and  death,  which  affects 
only  the  elements  or  tissues.  This  distinction  is 
much  more  apparent  in  animals  than  in  plants  on 
account  of  the  greater  degree  of  specialization  in 
the  former.  Ordinarily,  decay  and  disintegration 
in  the  tissues  promptly  follow  general  death,  but 
experimentally  we  may  avoid  this  contingency  if  we 
exclude  bacterial  invasion,^  and  such  a  piece  of 
tissue  may  be  kept  passively  "  alive  "  for  a  consider- 
able interval  of  time,  regaining  its  functions  when 
replaced  in  a  living  organism.     In  this  way  sections 

1  See  Chapter  IV. 


LIVING  SUBSTANCE  5 

of  blood  vessels  and  other  organs  have  been  cut  out 
and  later  replaced  in  the  same  or  other  animals 
without  injury.  By  keeping  such  a  tissue  at  a 
trifle  above  freezing  point  the  period  of  suspended 
vitality  may  be  extended  to  weeks  or  months.^ 
Recent  experimenters  have  shown  that  not  only  may 
the  pieces  of  excised  tissue  be  kept  passively  alive, 
but  that  under  proper  conditions  they  will  sprout 
and  grow  like  so  many  plant  cuttings.  It  is  only 
necessary  that  they  be  surrounded  by  a  nutritive 
medium  drawn  from  the  same  animal  from  which 
they  came,  and  that  they  be  kept  free  from  all  bac- 
teria. 

If  the  turtle  heart  in  the  experiment  described 
should  after  a  while  cease  to  beat,  but  later  begin 
to  do  so  again,  we  would  of  course  say  that,  like  the 
excised  tissues  just  described,  it  was  still  alive  during 
its  period  of  inactivity,  although  our  only  knowledge 
of  its  being  alive  is  derived  from  its  subsequent 
beating.  For,  we  say,  our  idea  of  life,  however 
vague  it  may  be,  does  not  admit  of  discontinuity. 
'Once  alive,  always  alive,  until  dead. 

The  experimental  physiologist  is  not  so  sure  of 
that.  Such  a  suspended  heart  muscle  of  the  turtle 
will  not  beat  except  in  the  presence  of  salt  (or  some 
sodium  compound).  But  in  pure  salt  solution  it 
stops  beating.  The  pure  salt  acts  as  a  poison.  We 
might  now  consider  the  muscle  dead,  were  it  not 

1  We  find  an  analogous  instance  in  Nature  in  the  fact  that  many 
seeds  will  retain  their  vitality  unimpaired  for  years,  until  proper  condi- 
tions of  warmth  and  moisture  cause  them  to  sprout  and  grow. 


6  GENERAL   BIOLOGY 

for  the  fact  that  if  we  add  a  little  calcium  chloride 
to  the  salt  solution,  the  heart  begins  to  beat  again. 
The  calcium  has  neutralized  the  ill  effect  of  the 
sodium.  If,  then,  contracting  is  any  criterion  of 
whether  the  heart  is  alive  or  not,  its  life  would  seem 


Fig.  1.  —  A  tardigrade  (Macrobiotus)  :    a,  in   the  creeping  active  con- 
dition ;  b,  dried,  in  the  state  of  apparent  death. 


to  depend  upon  the  presence  or  absence  of  something 
quite  outside  the  living  matter  at  all ! 

Under  perfectly  natural  conditions  such  a  state 
of  "ceasing  to  live  "  and  then  resuming  life  again  is 
not  uncommon.  Some  animals  that  live  in  shallow 
puddles  exposed  to  the  chance  of  drying  up  are  capa- 


LIVING  SUBSTANCE  7 

ble  of  drying  up  also,  and  remaining  in  an  apparently 
lifeless  condition  for  long  periods  of  time,  blown 
about  in  the  dust  by  the  winds  (fig.  1).  Falling  in 
a  favorable  spot  where  there  is  sufficient  water,  they 
"  come  to  life  again  "  and  resume  their  activity 
as  if  it  had  never  ceased.  The  excessively  minute 
*'  germs  "  of  bacteria  keep  the  race  in  existence  in 
this  way. 

In  seeking  an  analogy  to  these  phenomena  a 
German  scientist,  Preyer,  has  compared  the  plant 
or  animal  to  a  clock,  which  goes  through  its  charac- 
teristic movements  so  long  as  the  energy  in  its 
mainspring  lasts.  It  may  be  stopped  and  remain 
so  until  its  pendulum  is  set  swinging  again,  in  which 
case  it  may  be  compared  with  a  fertile  seed.  But 
if  its  mainspring  be  broken  or  if  it  run  down,  even 
though  externally  it  be  just  the  same  in  appearance, 
it  no  longer  "  goes."  Some  such  a  difference  as 
this  may  exist,  not  to  push  the  comparison  too 
far,  between  an  organism  in  which  life  is  merely 
suspended  and  one  that  is  dead.^ 

Our  search,  therefore,  for  the  answer  to  the  ancient 

1  Waller  has  shown  that  in  such  forms  of  living  substance  as  nerves, 
which  do  not  contract  or  give  any  visible  evidence  of  life  or  death,  it  is 
possible  by  galvanometric  test  to  show  that  a  "live"  nerve  deflects  the 
needle,  whereas  a  "dead"  one  does  not;  in  other  words  that  the  electric 
response  is  a  very  delicate  and  accurate  sign  of  life.  By  this  means  he 
claims  to  have  been  able  to  mark  the  "beginning  of  life"  in  an  incubating 
hen's  egg.  A  Hindu  physiologist.  Professor  Bose,  has  claimed,  on  the 
basis  of  very  careful  experimental  work,  that  this  electrical  sign  of  life 
is  dependent  upon  the  "molecular  mobility"  of  the  matter,  and  that  it 
disappears  when  "molecuj^r  fixation"  or  strains  ensue.  Herein  may  be, 
possibly,  the  simple  difference  between  living  and  dead  matter. 


8  GENERAL  BIOLOGY 

conundrum,  "  What  is  life  ?  "  so  long  as  we  attempt 
to  solve  it  by  processes  of  analysis,  leads  us  up  a 
blind  alley  no  matter  what  clue  we  follow.  Yet, 
in  spite  of  our  difficulty  in  defining  living  matter, 
we  recognize  the  existence  of  it  as  something  real 
and  not  imaginary,  and  when  we  compare  the  in- 
numerable kinds  of  living  things  from  the  standpoint 
of  their  physical  and  chemical  composition,  we  find 
that  they  all  have  much  in  common;  that  life, 
whatever  its  metaphysical  aspects,  has  also  a  mate- 
rial basis,  a  "  life  stuff  "  or  living  substance.  This 
living  substance  has  received  various  names,  but 
that  which  is  most  commonly  used  is  Protoplasm.^ 
This  is  what  Huxley  called  in  enduring  phrase  "  the 
physical  basis  of  life." 

Chemistry  of  Protoplasm.  —  In  studying  the 
physics  and  chemistry  of  protoplasm  we  find  that 
it  is  exceedingly  complex.  But  its  complexity 
arises  from  the  almost  infinite  combinations  and 
permutations  of  a  very  limited  series  of  chemical 
elements.  Carbon,  hydrogen,  oxygen,  nitrogen,  — 
these  we  find  in  all  protoplasm,  and  they  constitute 
its  bulk.  Sulphur  and  phosphorus  are  also  always 
present,  but  in  very  much  smaller  quantities,  and 
usually  chlorine,  potassium,  sodium,  magnesium, 
calcium,  and  iron  as  well.  Many  other  elements 
occur  normally,  though  rarely,  in  the  protoplasm  of 
certain  animals  and  plants.     Iodine  occurs  in  sea- 

^  Bioplasm,  a  term  used  by  many  English  authors,  is  perhaps  prefer- 
able, but  protoplasm  is  firmly  established  in  the  literature  of  biology. 


LIVING  SUBSTANCE  9 

weeds  and  in  the  thyroid  gland  of  certain  animals. 
Zinc  and  manganese  seem  to  be  normal  constituents 
of  the  tissues  of  some  mollusks.  In  other  words, 
protoplasm  is  not  a  definite  chemical  substance  or 
compound,  like  quartz  or  salt  or  starch,  but  is  some- 
times one  thing,  sometimes  another.  Rather,  it  is 
a  mixture  of  various  things,  all  of  them,  however, 
of  an  infinite  complexity  of  mutual  relations,  — 
"  a  mixture,  but  certainly  no  jumble."  The  word 
"protoplasm,"  then,  is  a  sort  of  group  name  covering 
a  multitude  of  different  sorts  of  such  chemical  mix- 
tures, as  many  as  there  are  different  manifestations 
of  life  phenomena. 

It  has  been  just  said  that  the  bulk*  of  all  kinds  of 
protoplasm  is  made  up  of  carbon,  hydrogen,  oxygen, 
and  nitrogen.  These  exist  in  elaborate  combina- 
tions, of  which  the  carbon  atom  seems,  as  a  rule, 
to  form  the  heart  or  foundation.  The  older  "or- 
ganic chemistry,"  or  the  chemistry  of  organisms  and 
their  products,  has  become  the  "  chemistry  of  the 
carbon  compounds." 

An  exception  must  be  made  to  the  statement 
that  the  combinations  of  the  four  elements  cited 
are  always  complex.  One  of  the  simplest  of  all 
compounds,  water  (H2O),  is  a  necessary  constit- 
uent of  living  matter.  The  percentage  of  water  in 
all  protoplasm  is  high.  Muscles  are  three  fourths 
water,  even  bones,  nearly  one  fourth,  and  in  the 
jellyfishes  of  the  open  sea,  that  which  is  not  water 
is  but  one  per  cent  or  less  of  the  total  bulk.  With 
these  facts*  in  mind,  one  is  inclined  to  think  of  living 


10  GENERAL  BIOLOGY 

organisms  as  liquids  that  contain  solid  matter  rather 
than  as  solids  with  a  percentage  of  liquids.  The 
large  amount  of  water  in  protoplasm  is  a  very  im- 
portant and  significant  feature  of  its  make-up, 
since  it  affords  a  means  for  the  transfusion  of  sub- 
stances from  one  part  of  the  animal  or  plant  to 
another,  and  gives  the  organism  a  certain  necessary 
plasticity  as  well.^ 

The  combinations  of  carbon  (C),  oxygen  (O), 
hydrogen  (H),  and  nitrogen  (N)  that  make  up  the 
bulk  of  protoplasm  fall  into  three  great  groups  or 
classes,  the  proteins  or  albumens,  the  carbohydrates 
(sugar  and  starches),  and  the  fats.  These  three 
groups  are  much  more  easily  described  than  defined. 

The  Proteins  are  found  in  all  protoplasm  and  are 
indispensable  to  the  processes  of  life.  They  consti- 
tute a  large  and  diverse  group  differing  widely  one 
from  another,  but  all  sharing  certain  group  characr 

1  Recent  advances  in  physical  chemistry  have  thrown  much  light  on 
the  physical  and  chemical  processes  of  protoplasm.  It  has  been  dis- 
covered that,  in  great  dilution,  many  chemical  substances  tend  to  dis- 
sociate into  ions,  as  e.g.  NaOH  into  Na  and  OH,  each  ion  consisting,  in 
the  current  explanation,  of  an  atom  or  atomic  group  bearing  a  charge  of 
negative  or  positive  electricity.  As  a  rule,  it  is  only  in  this  dissociated 
condition  that  atoms  are  active  in  combining  with  one  another.  It  has 
been  found  by  experiment  that  the  effect  of  certain  salts  on  protoplasm 
(for  instance  the  poisonous  action  of  the  heavy  metals)  is  in  direct  pro- 
portion to  the  ionization  of  the  salts.  It  has  been  shown  also  that  cer- 
tain salts  are  absolutely  essential  to  life  processes,  although  the  amount 
required  may  be  very  minute.  A  fresh-water  crustacean,  Gammarus, 
according  to  W.  Ostwald,  inevitably  dies  if  placed  in  absolutely  pure 
distilled  water,  but  will  live  indefinitely  if  a  trace  of  common  salt 
(NaCl)  be  added.  The  amount  necessary  is  only  eight  ten-thousandths 
of  a  gram  (twelve  thousandths  of  a  grain,  Troy)  to  a  liter  of  water. 


LIVING  SUBSTANCE  11 

teristics.  Among  these  are  the  invariable  presenee — 
of  nitrogen  along  with  the  carbon,  hydrogen^  and 
oxygen,  and  a  very  large  and  complex  molecule, 
which  is  always  Isevo-rotatory,  i.e.  turns  the  rays  of 
polarized  light  to  the  left,  ^jid  contains  sometimes 
thousands  of  atoms. ^  About  half  the  weight  is  car-_ 
bon  and  15  per  cent  to  18  per  cent  nitrogen. 

A  giant  protein  molecule  is  not  exactly  a  unit  in 
itself,  but  is  usually  an  aggregation  of  smaller  atomic 
groups  or  other  protein  molecules  weakly  held  to- 
gether by  the  bond  of  "  chemical  affinity,"  just  as 
a  village  of  five  hundred  inhabitants  may  be  thought 
of  as  made  up  not  exactly  of  that  many  individuals, 
but  of  one  hundred  families  composed  in  turn  of 
an  average  of  five  persons  each.  The  atomic 
groups  may  freely  break  away  from  the  protein 
molecule  or  be  added  to  it,  and  in  this  instability  lies 
the  great  significance  of  the  presence  of  the  proteins 
in  all  living  matter.  (See  chapter  on  Metabolism.) 
As  examples  of  nearly  pure  protein  may  be  mentioned 
lean  meat  fiber  and  white  of  egg  (albumen). 

J.  Loeb  and  Pauli  have  called  attention  to  the 
strong  probability  that  the  proteins  in  animal  proto- 
plasm are  united  with  the  ionized  ^  inorganic  salts 
to  form  "ion-protein  compounds."  This  hypothesis 
accounts  for  many  otherwise  inexplicable  phenom- 
ena, and  explains  the  importance  of  ev^n  extremely 
minute  quantities  of  certain  salts  in  life  processes. 

^  An  analysis  of  the  blood  pigment  of  the  horse  has  yielded  the  fol- 
lowing formula :  C7i2Hii3oN2i40245FeiS2. 
2  See  page  10. 


n  GENERAL  BIOLOGY 

'The  Carbohydrates  (as  also  the  Fats)  lack  the 
nitrogen  characteristic  of  protein.  They  contain 
only  carbon,  hydrogen,  and  oxygen,  the  latter  two 
always  present  in  the  proportion  found  in  water, 
twice  the  number  of  hydrogen  atoms  as  of  oxygen. 
They  are  simpler  in  structure  than  the  proteins,  but 
like  them  may  be  combined  into  molecular  aggre- 
gates of  higher  degrees  of  complexity.  The  simple 
sugars  or  monosaccharids,  of  which  dextrose  or  glucose 
is  the  most  familiar,  have  the  formula  C6H12O6. 
By  combining  two  molecules  of  a  monosaccharid 
with  the  loss  of  a  molecule  of  water  ^  a  disaccharid 
may  be  formed,  of  which  the  most  familiar  example 
is  cane  sugar  (sucrose).  Under  the  influence  of 
yeast  a  monosaccharid  will  break  up  into  carbon 
dioxide  and  alcohol,  a  process  known  as  fermenta- 
tion. By  continuing  the  addition  of  monosaccharid 
molecules,  one  to  another,  each  time  with  the  loss 
of  a  molecule  of  water,  more  complex  sugars,  the 
polysaccharids,  are  formed.  These  are  the  starches 
and  dextrines,  and,  most  familiar  of  all,  cellulose 
and  woody  fiber.  The  carbohydrates  in  general 
are  more  abundant  in  plants  than  in  animals,  al- 
though one  of  them,  glycogen,  which  is  found  abun- 
dantly in  the  liver  and  muscles  of  higher  animals, 
is  of  very  great  importance  in  animal  nutrition. 

The  Fats  contain  the  same  elements  found  in 
the  carbohydrates,  C,  H,  and  O,  but  in  different 
proportions  and  arrangements.  In  every  case  they 
are  the  result  of  the  combination  of  an  acid  with 

1  CeHizOe  +  CeHizOe  =  C12H22O11  +  H2O. 


LIVING  SUBSTANCE 


13 


glycerine.  The  acids,  with  few  exceptions,  belong 
to  the  "  fatty-acid "  series.  Three  molecules  of 
fatty  acid,  not  necessarily  of  the  same  kind,  combine 
with  one  of  glycerine  to  form  a  fat.  The  cleavage 
and  recombination  of  these  two  component  parts  of 
a  fat  (the  fatty-acid  element  and   the  glycerine)  is 


Fig.  2.  —  Milk  under  the  microscope,  showing  the  nature  of  an 
emulsion.  The  spheres  are  fat  globules,  the  dark  rods  lactic-acid- 
forming  bacteria.  (From  "  Elements  of  Biology,"  copyright,  1907,  by 
George  William  Hunter.  —  Permission  of  the  American  Book  Co., 
publishers.) 

easily  accomplished.  When  the  fatty-acid  portion 
combines  with  an  alkali,  it  forms  a  soap.  Fats  are 
insoluble  in  water,  but  readily  soluble  in  ether,  ben- 
zine, etc.,  —  a  fact  made  use  of  in  the  cleaning  of 
clothing.  They  may  be  shaken  up  in  alkaline  water 
until  the  particles  become  very  finely  divided  and 
remain  in  suspension,  forming  an  emulsion.  The 
most  familiar  example  of  such  an  emulsion  is  milk. 


14  GENERAL  BIOLOGY 

Physical  Structure  of  Protoplasm.  —  Proteins 
are  not  soluble  in  water  in  the  usual  sense,  that 
is,  they  do  not  make  a  clear  solution  as  do 
sugar  and  salt.  They  do  absorb  a  great  quantity 
of  water,  however,  and  swell  up  enormously.  In 
the  presence  of  large  amounts  of  water  they  m^ 
become  very  finely  divided  and  form  permanent 
aqueous  suspensions,  wKich  differ  from  true  solu- 
tions in  that  they  will  not  diffuse  through  vegetable 
parchment  or  animal  membranes.  Such  substances 
are  usually  known  as  "  colloids,"  in  contrast  to 
"  crystalloids  "  or  substances  which  do  diffuse 
through  such  membranes.  Another  characteristic 
of  colloids  is  their  property  of  coagulating  or  "  set- 
ting," a  familiar  example  of  which  may  be  observed 
in  the  hardening  of  the  white  of  an  egg  in  the  pro- 
cess of  boiling.  Such  coagulation  may  be  produced 
by  heat,  electric  currents,  dehydration,  and  chemical 
reagents.  Certain  classes  of  colloids,  like  the  egg 
albumen,  are  unalterable  when  once  coagulated,  and 
are  known  as  irreversible.  Others  may  be  brought 
back  to  the  fluid  state  any  number  of  times.  Such 
substances,  of  which  gelatin  is  an  example,  are  called 
reversible.  The  essential  difference  between  "  solu- 
tions "  of  colloids  and  of  crystalloids  consists  in 
the  fact  that  the  particles  of  the  former  are  larger 
and  are  enveloped  in  a  film  of  water,  whereas  true 
solution  involves  the  separation  of  the  crystalloid 
into  its  molecules  or  even  into  its  ions,  in  which  con- 
dition the  particles  obey  the  law  of  gases  and  the 
solution    exerts    pressure    (osmotic    pressure)  in  all 


LIVING  SUBSTANCE  15 

directions.  The  colloidal  nature  of  the  proteins, 
therefore,  is  probably  to  be  attributed  to  the  great 
size  of  the  molecules  of  which  they  are  composed, 
aggregates  which  in  fact  are  not  true  molecules  but 
composites  of  other  smaller  aggregates.  Chemists 
refer  to  this  welding  together  of  molecular  aggre- 
gates as  polymerization.  As  we  shall  see  later,  the 
process  of  animal  digestion  involves  merely  the 
breaking  up  of  these  aggregates  into  others  of  lesser 
degree,  small  enough  to  diffuse  through  the  lining 
membranes  of  the  alimentary  canal. 

Protoplasm,  being  composed  largely  of  proteins, 
is  thus  colloidal  in  its  physical  make-up.  But  the 
examination  of  living  protoplasm  with  the  high 
powers  of  the  microscope  reveals  a  structure  much 
more  complex  than  may  be  found  in  a  mere  lump 
of  non-living  colloid.  Living  substance  has  a 
characteristic  physical  structure  of  its  own,  to  ex- 
plain which  several  theories  have  been  advanced. 
According  to  one,  the  essential  basis  of  protoplasmic 
structure  is  granular,  and  granules  are  certainly  to 
be  found  in  protoplasm.  Others  find  fibrils  like 
detached  threads,  others  see  a  skein  or  reticulum  in 
the  meshes  of  which  more  watery  substances  are 
held.  The  view  first  advocated  by  Butschli  is, 
however,  the  one  most  commonly  held  by  biologists. 
According  to  this  theory,  protoplasm  has  the  struc- 
ture of  a  foam  in  which  the  denser  parts  surround 
the  lighter  as  the  film  of  water  does  the  air  in  soap 
bubbles.  Perhaps  a  more  accurate  object  of  com- 
parison would  be  a  fine  emulsion.     In  an  emulsion, 


16  GENERAL   BIOLOGY 

instead  of  a  gas  and  liquid  (as  in  soapsuds),  we  have 
two  liquids  of  different  densities  and  qualities,  form- 
ing what  is  known  as  a  diphasic  system.  According 
to  Butschli  protoplasm  is  such  an  emulsion,  com- 
posed on  the  one  hand  of  substances  insoluble  in 
water  which  are  highly  viscous  or  sticky,  and  on  the 
other  hand  of  a  watery  medium  supporting  the 
various-sized  particles  of  the  former. 

Owing  to  the  fact  that  the  refractive  indices  of 
the  different  components  of  protoplasm  are  nearly 
the  same,  it  is  very  difficult  to  see  its  structure  in 
the  unaltered  living  state.  Biologists  have  recourse, 
therefore,  to  *'  fixing  "  or  coagulating  the  protoplasm 
with  various  poisons  and  dyeing  it  with  aniline  or 
other  colors.  Under  such  circumstances  protoplasm 
appears  to  have  a  skein  or  net  structure.  But  this 
has  been  interpreted  as  being  merely  the  appearance 
of  particles  caught  and  held  by  surface  tension  — 
the  films  of  the  bubbles,  so  to  speak,  which  when 
set  and  viewed  in  connection  with  those  of  adjacent 
bubbles  appear  to  form  a  continuous  layer  or  thread. 
This  view  is  justified  by  the  fact  that  artificial  emul- 
sions, subjected  to  the  same  processes  of  fixation 
and  staining  employed  in  the  study  of  protoplasm, 
show  a  strikingly  similar  appearance  to  that  of 
living  substance.  More  recent  investigations,  in- 
volving the  dissection  of  living  protoplasm  under 
the  highest  powers  of  the  microscope,  seem  to  point 
to  the  conclusion  that  Butschli's  conception  may 
require  certain  modification.  The  physical  "  phases  " 
in  which  protoplasm  consists  appear  to  be  all  col- 


LIVING  SUBSTANCE  117 


loidal,  differing  only  in  the  amount  of  water  which 
they  absorb,  an  amount  which  may  be__i:apidly 
altered  under  varying  circumstances. 

Organization  of  Protoplasm.  —  Not  only  is  pro- 
toplasm, chemically  considered,  a  mixture  of  a  great 
variety  of  substances,  but  this  aggregate  of  ma- 
terials composing  a  given  mass  of  living  matter  in 
one  kind  of  animal  or  plant  differs  from  that  of 
another.  Moreover,  the  same  mass  of  protoplasm 
is  constantly  changing  with  regard  to  the  substances 
composing  it,  thus  making  impossible  anything 
like  a  fixed  and  definite  picture  of  its  exact  composi- 
tion. But  it  is  also  true  that  it  is  not  so  much  the 
substances  composing  it,  as  the  relations,  both  physi- 
cal and  chemTcaiPthat  these  bear  to  one  another 
that  determines  the  character  of  the  protoplasm.  A 
comparison  may  make  this  clearer.  A  watch  *  is  a 
delicately  constructed  and  complicated  mechanism 
of  many  parts  which  by  their  action  in  moving  the 
hands  in  certain  fixed  relations  of  time"  and  space 
enables  us  thereby  to  tell  the  time  of  day.  It  is 
possible  for  any  one  to  take  such  an  instrument  apart 
and  make  a  little  pile  of  the  wheels,  screws,  and 
springs,  but  when  he  has  done  so,  the  mass  of  metal 
and  jewels  that  he  holds  in  his  hand  is  no  longer  a 
watch  and  cannot  be  made  to  serve  its  original 
purpose.  The  very  apparent  reason  is  that  the 
inherent  quality  of  a  watch,  by  virtue  of  which  it  is 
a  watch,  that  is,  a  timekeeper,  is  involved  not  only 
in  the  parts  composing  it,  but  in  their  relations  to 


18  GENERAL  BIOLOGY 

each  other  as  well,  and  such  a  mechanism  will  run 
correctly  only  when  every  part  is  in  its  proper 
place  and  adjusted  carefully  to  every  other  part  with 
reference  to  the  joint  action  of  the  whole. 

It  is  so  with  protoplasm.  However  we  may  de- 
fine life  it  is  certainly  true  that  the  property  of 
protoplasm,  by  virtue  of  which  it  is  "  living  matter," 
is  bound  up  in  the  interrelations  of  the  various  parts 
composing  it.  And  just  as  a  machine  must  be 
properly  assembled,  to  use  a  technical  phrase,  so 
protoplasm  does  not  exist  as  living  substance  except  it 
be  organized.  Destroy  or  fundamentally  alter  this  or- 
ganization and,  although  we  may  get  the  same  chem- 
ical analysis  of  the  material  and  the  same  weight 
of  substance,  it  is  no  longer  alive.  But  it  must  be 
noted  that  the  comparison  with  the  watch  cannot 
be  pushed  too  far,  for,  unlike  a  rigid  mechanism, 
protoplasm  is  extremely  plastic  and  capable  of 
adjusting  itself  to  very  wide  ranges  of  structural 
alteration  without  ceasing  to  be  alive.  Indeed  the 
living  organism  is  constantly  so  adjusting  itself 
in  response  to  external  conditions,  —  a  phenomenon 
which  many  hold  to  be  the  fundamental  and  charac- 
teristic fact  of  life  itself. 

The  Cell.  —  The  difference  between  an  oak  leaf 
and  a  waxen  image  of  one  is  not  alone  a  difference 
in  the  chemical  substances  composing  the  two. 
Should  we  cut  a  thin  slice  from  the  wax  leaf  and 
examine  it  under  a  microscope  we  could  distinguish 
nothing   to   mar   its   homogeneity.     Should   we   do 


LIVING   SUBSTANCE 


19 


the  same  with  the  leaf  itself  we  would  find  at  once 
that  the  leaf  bears  to  its  waxen  counterfeit  the  same 
relation  that  a  mosaic  figure  does  to  a  photograph. 
Instead  of  being  homogeneous  in  structure  it  is 
composed  of  innumerable  little  units,  —  the  aggre- 
gate of  which  makes  up  the  mass  of  the  leaf. 

These  structural  units  of  organization  of  proto- 
plasm are  called  cells.     The  word  owes  its   deriva- 


FiG.  3.  —  Section  of  a  leaf,  showing  its  cellular  composition  :  a,  a 
breathing  pore  or  stoma  ;  b,  upper  layer  of  "  palisade  cells  "  which  con- 
tain most  of  the  chlorophyll ;  c,  epidermal  cell.     (Bailey.) 

tion  to  the  fact  that  the  discoverer  of  the  first  cells 
described  found,  in  examining  a  thin  slice  of  cork, 
that  the  cork  was  made  up  of  little  boxes  like 
the  cells  of  the  honeycomb.  Similar  observations 
were  made  later  on  a  great  variety  of  tissues  until 
it  was  established  that  all  plants  and  animals  are 
composed  of  cells  as  structural  units  in  much  the 
same  way  that  a  house  is  built  of  individual  bricks 
or  stones. 

Of  course  the  shapes  and  structures  of  these 
units  vary  greatly  in  accordance  with  the  kinds  of 
tissues  in  which  they  are  found  or  the  activities  they 


20 


GENERAL  BIOLOGY 


reveal,  but  there  are  certain  structures  that  are 
common  to  all  cells,  and  they  may  be  considered 
fundamental  in  cell  organization.  Postponing  for 
a  moment  the  consideration  of  the  outer  limit  of  the 


Fig.  4.  —  Diagram  of  a  composite  cell.  M,  metaplasmic  bodies  ;  v, 
vacuoles  ;  pi,  plastids  (chloroplasts)  ;  c,  attraction  sphere  inclosing  a 
double  centrosome  ;  nu,  true  nucleolus  ;  chr,  chromatin  network  ;  I,  linin 
network  ;  k,  karyosome  or  chromatin  nucleolus.  Vacuoles  are  especially- 
characteristic  of  plant  cells.  The  chloroplasts  are  found  only  in  plant 
cells.  They  are  capable  of  independent  growth  and  reproduction.  It  is 
these  that  give  the  green  color  to  leaves. —  (From  Wilson.) 

cell  or  cell-wall,  we  find  that  all  cells  agree  in  having 
the  protoplasm  composing  them  differentiated  into 
two  parts,  —  a  more  or  less  central  one,  —  the  nucleus, 
often  rounded  in  outline  and  somewhat  dense  in  con- 
sistency, which  is  surrounded  by  a  less  dense  area,  — 


LIVING  SUBSTANCE  21 

the  cytoplasm.  Nucleus  plus  cytoplasm  together 
make  up  the  cell. 

Bounding  the  cytoplasm  there  may  be  a  definite, 
often  thickened  cell-ivall.  This  is  especially  charac- 
teristic of  plant  tissues,  in  which  the  cell- wall  becomes 
very  thick  and  rigid  through  the  deposit  of  various 
carbohydrates  (pectin,  cellulose) .  It  is  the  latter  (or 
its  derivative,  lignin)  that  gives  its  characteristic 
rigidity  and  hardness  to  wood.  In  animals,  on  the 
other  hand,  in  only  one  rather  obscure  group  ^  does 
cellulose  occur,  and  it  is  the  exception  rather  than 
the  rule  for  the  cell-wall  to  attain  any  considerable 
thickness  or  prominence.  In  many  animal  cells  there 
is  no  cell- wall  at  all,  —  the  viscosity  and  surface  ten- 
sion of  the  mass  of  protoplasm  holding  it  together. 

The  nucleus  and  cytoplasm  are  found  by  delicate 
tests  to  be  chemically  different,  the  former  combining 
more  readily  with  basic  and  the  latter  with  acid  sub- 
stances. The  cytoplasm  often  contains  vacuoles 
filled  'with  a  watery  fluid  or  with  different  sorts  of 
non-living  substances,  such  as  crystals  of  silica 
(that  give  the  knife-edge  to  certain  kinds  of  grass), 
chlorophyll  bodies  (that  give  the  green  color  to 
plant  leaves),  starch  grains  (as  in  the  potato), 
yolk  granules  (such  as  make  up  the  bulk  of  the 
yellow  of  a  hen's  e^g),  and  various  other  substances. 
Such  substances,  being  non-living  material,  manu- 
factured by  the  cell,  are  called  metaplasm  in  contra- 
distinction to  the  living  protoplasm. 

In  the  cytoplasm  is   also  found  the  centrosome, 

1  The  Ascidians  or  "Sea  Squirts." 


22 


GENERAL  BIOLOGY 


a  structure  that  appears  only  at  certain  periods  of 
the   cell's   activity   and   is   either   invisible   or   non- 


FiG.  5.  —  Various  kinds  of  cells  :  .4,  female  germ-cell,  ovum  of  the 
cat ;  B,  male  germ-cell,  spermatozoon  of  a  snake.  Coluber ;  C,  ciliated 
epithelium  from  the  digestive  tract  of  a  moUusk,  Cyclas ;  D,  cartilage  of 
a  squid ;  E,  striated  muscle  fiber  from  an  insect  larva,  Corydalis 
cornutus ;  F,  a  nerve  cell  from  the  cerebellum  of  man. —  (Dahlgren 
and  Kepner.) 

existent    throughout    the    greater   part  of    cell   life. 
(See  under  Cell  Division,  Chapter  IV.) 
The    nucleus    (sometimes    called    karyoplasm    in 


LIVING  SUBSTANCE  23 

contrast  to  cytoplasm),  owing  to  the  fact  that  it 
has  about  the  same  refractive  index  as  the  cyto- 
plasm, is  usually  almost  or  quite  invisible  in  living 
cells,  and  must  be  fixed  and  stained  before  it  can  be 
easily  seen.  Its  outline  is  always  sharply  distinct 
from  the  adjacent  cytoplasm,  and  often  a  limiting 
membrane  seems  to  be  present,  though  the  presence 
of  the  latter  in  all  living  cells  is  not  definitely  estab- 
lished. 

Within  the  nucleus  the  protoplasm  is  further  dif- 
ferentiated into  two  substances,  —  one  that  stains 
very  readily  with  most  dyes,  and  for  that  reason  is 
called  chromatin,  and  another  (the  linin)  that  stains 
with  great  diflficulty,  and  looks  like  a  sort  of  network 
or  scaffolding  supporting  the  chromatin.  Both 
chromatin  and  linin  are  surrounded  by  a  watery 
transparent  fluid  sometimes  termed  hyaloplasm. 
In  many  cells,  especially  egg-cells  of  animals,  a 
nucleolus  is  prominent,  —  a  rounded  aggregation 
of  chromatin  material  which  is  found  to  be  chem- 
ically different  from  the  true  chromatin,  but  the 
nature  and  function  of  which  has  never  been  clearly 
understood. 

The  nucleus  owes  its  acid  nature  to  the  chromatin, 
which  is  largely  made  up  of  some  form  of  nucleic 
acid,  a  complex  substance  having  a  characteristic 
percentage  of  phosphorus.  When  fixed  and  stained, 
the  chromatin  usually  appears  in  the  form  of  gran- 
ules of  either  large  or  minute  dimensions,  sometimes 
arranged  in  the  form  of  a  skein  or  network,  with 
"  knots  "  at  the  intersections.     Only  in  cells  that 


24 


GENERAL  BIOLOGY 


are  in  process  of  reproduction  is  there  any  definite- 
ness  of  form,  shape,  or  number  to  the  chromatin 
aggregates. 

Usually  we  find  only  one  nucleus  in  a  cell,  but 
sometimes  many  nuclei  occur,  scattered  through  the 
cytoplasm.  In  plants  the  cell- wall  is  an  important 
part  of  the  cell,  since  it  affords  the  necessary  rigidity 

and  strength   to 

Lmmn' 


0     0 


m. 


ipil 


W^':Q^'^.^ 


B 


Fig.  6.  —  Sections  of  the  outer  epithelium 
(skin)  of  a  creeping  Ctenophore  (Cceloplana)  : 
A,  a  ciliated  region  in  which  the  cells  are  pro- 
vided with  locomotor  organs  in  the  form  of 
vibratile  cilia,  a  gland-cell  at  the  left ;  B,  a 
non-ciliated  region  of  the  same  epithelium ; 
the  cell-walls  are  barely  indicated  (except  in 
the  gland  cells) ;  nuclei  are,  however,  abundant. 


the  plant,  func- 
tioning in  this 
way  much  as  the 
skeleton  does  in 
animals.  But  we 
find  that  the  pres- 
ence or  absence 
of  a  cell-wall  is 
conditioned 
largely  if  not  en- 
tirely by  such  a 
demand.  In  tis- 
sues where  there 
are  strains  and 
stresses     to     be 


borne,  cell-walls  develop  in  response  to  such  stimuli, 
but  where  they  are  not  necessary  they  do  not  appear, 
or  only  partially  develop.  Such  tissues,  however, 
are  found  to  have  as  many  nuclei  as  if  they  were 
cut  up  by  cell- walls  into  individual  cells.  They  con- 
sist, as  it  were,  of  a  mass  of  cells  *'run  together,"  and 
for  that  reason  are  called  ccenocytes  or  syncytia 
(singular,  syncytium).     The  presence  of  the  nucleus 


LIVING  SUBSTANCE  25 

seems  to  be  necessary  to  the  cytoplasm  in  order  for 
it  to  properly  carry  out  its  functions,   for  we  find 
that  a  fragment  of  a  cell,  provided  it  has  a  bit  of 
nuclear  matter  included,  will  continue  to  live,  whereas 
a  bit  of  cytoplasm  without  a  nucleus  soon  disintegrates 
and  dies.     The  nucleus,  in  other  words,  is  essential 
for  the  life  of  the  cell,  and,  accordingly,  since  the 
presence  or  absence  of  a  cell-wall  is  not  a  determin- 
ing character,  we  may  consider  the  essential  features  . 
of  a  cell  to  be  an  area  of  protoplasm  dominated  by  a  | 
nucleus.     The  cell  is  thus  a  dynamic  or  functionaL 
unit  rather  than  a  static  or  structural  one.     On  this 
account  the  term  energid    has  been  used  in  place 
of  the  less  accurate  word  "  cell." 


CHAPTER  II 

PRIMARY  FUNCTIONS  OF  THE  ORGANISM 

We  have  seen  that  protoplasm,  although  it  may 
be  resolved  into  a  mixture  of  various  complex  chem- 
ical substances  with  a  more  or  less  definite  physical 
structure,  does  not  exist  as  protoplasm  per  se,  but  is 
always  organized  in  certain  definite  relations,  one 
part  to  another  and  to  the  environment  of  the  whole. 
The  unit  of  this  organization  is  the  cell. 

The  individual  organism  usually  consists  of  many 
cells  linked  together  in  a  complex  whole,  but  the 
individual  may  also  subsist  in  a  single  cell.  In  the 
latter  case  we  speak  of  the  individual  as  a  Protozoan 
(or  Protophyte,  if  a  plant),  and  in  the  former  as  a 
Metazoan  (Metaphyte,  if  a  plant).  As  a  rule  the 
one-celled  organisms  (sometimes  spoken  of  collec- 
tively as  Protista)  are  simpler  both  in  structure  and 
in  function  than  those  of  many  cells.  Such,  how- 
ever, is  not  always  the  case.  The  most  complex 
of  the  Protozoa  is  as  specialized  in  organization  and 
functions,  if  not  more  so,  than  the  simplest  of  the 
Metazoa.  It  would  probably  be  more  accurate  to 
speak  of  the  Protozoa  as  non-cellular  rather  than  one- 
celled,  since  the  differences  between  the  two  groups 
are  qualitative  rather  than  quantitative,  i.e.  are  not 
based   on   the   number   of   cells   present.     So   from 

26 


PRIMARY   FUNCTIONS  OF  THE   ORGANISM  27 

another  point  of  view  we  may  consider  the  protozoan 
individual  to  be  comparable  to  the  metazoan  indi- 
vidual, with  the  difference  that  for  purposes  of  utility 
the  body  of  the  latter  is  subdivided  into  a  great 
number  of  dynamic  centers  (cells),  whereas  that  of  the 
former  is  not  so  divided. 


Fig.  7.  —  A  white  blood  corpuscle  (leucocyte)  of  the  frog  sketched 
at  frequent  intervals ;  from  a  to  m  the  temperature  was  gradually 
raised,  then  lowered  (n  to  p).  —  (From  Hertwig,  after  Engelmann.) 


In  the  metazoan  body,  however,  some  of  the 
cells  live  a  relatively  free  and  independent  existence, 
and  we  will  begin  our  study  of  the  phenomena  of 
cell  specialization  with  such  a  cell. 

In  the  blood  stream  cf  most  animals  are  to  be  found 


28  GENERAL   BIOLOGY 

free  cells,  "  corpuscles,"  floating  in  the  liquid  plasma. 
In  vertebrates  the  majority  of  these  cells  are  of 
definite  shape  and  are  the  carriers  of  the  charac- 
teristic red  pigment  of  blood.  Such  corpuscles  are 
also  found  in  a  few  worms  and  other  lower  organisms. 
In  addition  to  these  there  occurs  in  nearly  all  meta- 
zoans  in  which  J:here  is  any  blood  or  body-fluid 
another  sort  of  free  cell,  leucocytes  or  amoebocytes, 
distinguished  from  the  former  by  the  lack  of  red 
pigment  and  especially  by  the  absence  of  any  defi- 
nite shape  or  bodily  outline.  If  we  examine  the  blood 
of  an  earthworm  or  a  crayfish  with  a  microscope, 
we  may  study  these  cells  with  comparative  ease. 
If  the  plasma  containing  such  cells  be  kept  slightly 
warm,  these  leucocytes  will  be  found  to  change  form 
continually.  Short  processes  flow  out  from  the  cell- 
body  in  different  directions,  and  the  rest  of  the  pro- 
toplasm appears  to  flow  or  be  pulled  along  after  them. 
In  this  way  the  cell  is  able  to  progress  slowly  over 
the  slide  of  the  microscope  or  over  the  walls  of  the 
blood-vessels  in  which  it  normally  occurs.  In  other 
words  the  cell  possesses  the  function  of  locomotion. 
The  lobelike  processes  (called  pseudopodia  or  **  false 
feet  ")  are  protruded  at  any  part  of  the  cell-body 
or  on  several  parts  at  the  same  time.  This  function 
of  locomotion  is  therefore  unlocalized.  The  surface 
of  the  cell  appears  to  be  somewhat  sticky  (viscous) 
and  retains  a  hold  on  solid  objects.  When  the  cell 
is  creeping,  a  pseudopodium  sticks  in  this  way  to 
something  solid,  the  protoplasm  then  contracts,  and 
the  rest  of  the  cell  is  pulled  along  with  a  flowing 


PRIMARY  FUNCTIONS  OF  THE  ORGANISM    29 


movement.  The  actual  basis  for  these  creeping 
movements  is  thus  the  contractility  of  the  protoplasm 
constituting  the  cell. 

In  the  course  of  these  creeping  or  "  amoeboid  " 
movements  the  leucocyte  may  encounter  a  bit  of 
worn-out  tissue  or 
a  vagrant  bacterium. 
It  then  throws  out 
pseudopodia  on  both 
sides  of  the  object, 
which  flow  around  it, 
meet  beyond,  and  thus 
swallow  into  the  body 
of  the  cell  the  bac- 
terium or  tissue-frag- 
ment, together  with 
a  little  drop  of  the 
fluid  in  which  both 
are  floating.  (The 
bacterium  has  been 
ingested.)  While  in- 
side the  cell-body  of 
the  leucocyte,  certain 
changes  are  induced 
in  such  a  particle  by 
products  secreted  by 
the     leucocyte     that 

tend  to  dissolve  or  digest  it.  That  part  which 
cannot  be  so  digested  is  removed  by  the  reversal 
of  the  process  employed  in  swallowing  it,  i.e.  the 
leucocyte  creeps   on  and   leaves  it   behind,  —  it  is 


Fig.  8.  —  Phagocytes  (leucocytes) 
from  the  ccelomic  fluid  of  the  earth- 
worm :  A,  agglomeration  of  phagocj^es 
surrounding  a  foreign  body ;  B,  single 
leucocyte,  with  vacuoles.  —  (From  Sedg- 
wick and  Wilson,  after  Metchnikoff.) 


30  GENERAL   BIOLOGY 

egested.  The  digested  material  is  later  built  up  into 
the  substance  of  the  cell  protoplasm  by  the  process 
of  assimilation.  These  three  functions,  —  ingestion, 
digestion,  and  egestion,  —  like  that  of  locomotion, 
are  not  localized,  but  may  take  place  at  any  part  of 
the  body. 

If  bacteria  ^  be  introduced  into  the  body  of  an 
animal  at  any  point,  the  leucocytes  will  soon  be 
found  gathered  in  great  numbers  at  the  same  poinj^ 
It  has  been  shown  that  this  gathering  is  due  to  a 
mechanical,  i.e.  not  purposive,  "  attraction  "  exerted 
on  the  cells  by  the  chemical  substances  produced 
and  excreted  by  the  bacteria.  In  other  words,  the 
gathering  of  the  former  is  a  response  to  an  altered 
condition  of  the  medium  in  which  they  exist.  In 
the  same  way  the  cells  will  move  from  a  cool  region 
to  one  of  greater  warmth,  and  on  a  culture-slide, 
through  which  runs  a  weak  current  of  electricity, 
may  be  caused  to  gather  at  the  negative  pole.  A 
dead  leucocyte  will  not  respond  to  any  of  these 
"  stimuli."  The  response  is  therefore  a  function 
of  living  matter  and  is  spoken  of  as  a  result  of  its 
irritability.  Irritability  has  been  defined  as  "  the 
capacity  of  living  substance  of  reacting  to  changes  of 
environment  by  changes  in  the  equilibrium  of  its 
matter  and  its  energy." 

All  of  the  above  phenomena  may  be  observed  also 
in  Amoeba  proteus,  a  free-living  cell  found  in  slime 
and  stagnant  water.  Amoeba  is  an  independent 
organism,  whereas  the  leucocyte  is  one  cell  out  of  a 

1  e.g.  pus-forming  staphylococci. 


PRIMARY  FUNCTIONS  OF  THE  ORGANISM    31 

myriad  composing  an  organism.  Both,  however, 
exhibit  these  primitive  functions  inherent  in  living 
matter.  At  certain  times  Amoeba  secretes  about 
itself  a  tough  protecting  membrane  or  cyst  which  is 
the  product  of  protoplasmic  activity.     Within  this 


Fig.  9.  —  Amoeba  proteus  (much  enlarged)  :  1,  nucleus;  2,  contractile 
vacuole  ;  3,  pseudopodia  ;  4,  iood  vacuoles  ;  5,  grains  of  sand. —  (From 
Shipley  and  Mc Bride,  after  Gruber.) 


cyst  the  living  substance  tides  over  unfavorable 
periods,  reproducing  later  in  a  characteristic  way  to 
be  described  in  another  connection.  Under  favor- 
able circumstances  Amoeba  has  been  observed  to 
divide  in  two  half -cells  by  a  simple  constriction  and 
severance  of  the  cell.  This  process  is  preceded  by 
a  division  of  the  nucleus,  so  that  each  "  daughter 


32 


GENERAL  BIOLOGY 


cell  "  resulting  from  such  a  cleavage  has  half  the 
nuclear  material  of  the  original  parent  cell.  In 
this  way  the  number  of  individuals  is  greatly  in- 
creased, and  the  continuity  of  existence  of  the  race 
of  AmoeboB  insured.  Such  a  function  of  reproduc- 
tion also  accounts  for  the  great  number  of  leucocytes 
within  the  blood  stream  of  an  animal.  Although 
the  details  of  the  process  are  not  known  it  is  probable 
that  they  are  essentially  similar  to  what  has  just 
been  described  for  Amoeha. 

These,  then,  seem  to  be  primary  functions  of  living 
matter :  contractility,  assimilation,  irritability,  se- 
cretion, and  reproduction.  Of  nutrition  and  the  phe- 
nomena concerned  with  it  we  shall  have  more  to  say 
in  the  next  chapter. 


Specialization    in    Locomotor    Organs.  —  In    but 
few  cells   do   we  find   the  elementary  functions   of 

living  matter  so  evenly 
balanced  and  so  little 
specialized  as  in  the 
leucocyte  and  Amoeha. 
The  common  form  of 
Amoeha  just  described 
is  called  Amoeha   pro- 

FiG.  10.  — Amceba   angulata    (semi-  ^^^^      bccaUSC      OI      the 

diagrammatic),  showing    the   antenna-  f^ct  that  it  Constantly 
like    pseudopodium,   a,  which  vibrates  . 

back  and  forth  in  the  positions  marked  changes   its    shapC,  the 

1  and  2.  -  (After  Jennings.)  production   of   pSCudo- 

podia  being  unlocalized,  as  we  have  seen.    In  another 
species  of  Amoeha^  Amoeha  angulata,  there  has  been 


PRIMARY  FUNCTIONS  OF  THE  ORGANISM    33 


described  a  more  or  less  permanent  pseudopodium 
which  extends  from  one  edge  of  the  cell-body 
freely  into  the  water  "  and  waves  back  and  forth, 
serving  as  a  sort  of  feeler  or  antenna."  ^  In  yet 
another  form,  Masti- 
gamoeba,  there  is  a  per- 
manent lash  or  flagellum 
projecting  from  one  por- 
tion of  the  body,  the 
rest  of  the  creature  re- 
taining "  amoeboid  " 
movements.  In  the 
group  of  Protozoa  called 
"  Flagellata''  the  amoe- 
boid habit  is  not  found, 
but  the  animal  moves 
very  swiftly  by  the 
lashing  of  one  or  more 
permanent  whiplike  fla- 
gella.  In  these  three 
types  we  see  three  dif- 
ferent grades  of  special- 


FiG.  11.  — Mastigamoeha  aspera: 
f,  flagellum ;  p,  pseudopodium.  — 
(From  Calkins,  after  Schultze.) 


1  In  the  comparisons  that  follow,  the  reader  must  not  understand  that 
one  type  has  been  transformed  into  another  in  any  way  whatever. 
The  different  steps  have  been  arranged  side  by  side  much  as  one  might 
form  an  exhibit  of  different  models  of  the  telephone  or  the  phonograph 
from  the  first  crude  type  to  the  modern  improved  machine.  In  one 
sense,  though  not  in  a  material  or  genetic  sense,  the  perfected  phono- 
graph has  been  derived  from  the  earlier  model.  In  the  case  of  specializa- 
tion of  cells,  however,  as  we  shall  see  from  the  consideration  of  differ- 
entiation in  development  there  is  often  a  very  direct  genetic  relationship 
between  cells  of  a  specialized  type  and  those  of  the  most  generalized 
types. 


34 


GENERAL   BIOLOGY 


m^ 


ization,  in  each  one 
of  which,  compared 
with  Amoeba proteusy 
there  has  been 
brought  about  a 
very  much  greater 
efficiency  in  move- 
ment through  a  con- 
centration of  effort 
and  the  locaHza- 
tion  of  the  organ 
of  locomotion. 

In  another  group 
of  the  Protozoa,  not 
only  the  function  of 
movement,  but  also 
that  of  ingestion  has 
become  more  or  less 
specialized.  In  the 
Ciliates,  of  which 
Paramecium  is  a" 
typical  though  not 
the  most  generalized 
example,  the  animal 
swims  rapidly  by 
means  of  bristle-like 
cilia  developed  all 
over  the  cell-body. 
These  are  practically  all  alike,  and  consequently  the 
movements  of  the  animal  are  very  uniform  and  cir- 
cumscribed.    The   cilia   along   the   oral   groove   are 


Fig.  12.  —  Paramecium  viewed  from 
the  oral  surface :  L,  left  side ;  R,  right 
side;  an,  excretory  area  ("anus");  ec, 
ectosarc  ;  f.v.,  food  vacuoles  ;  g,  gullet ; 
m,  mouth  ;  ma,  macronucleus  ;  mi,  micro- 
nucleus  ;  o.g.,  oral  "groove  ;  p,  cuticle  ;  tr., 
trichocyst  layer.  —  (From  Jennings.) 


PRIMARY  FUNCTIONS  OF  THE  ORGANISM    35 

somewhat  heavier  tlnd  are  employed  to  force  food 
particles  into  the  so-called  gullet  to  a  point  where 
they  are  ingested.  Thus,  in  comparison  with  Amoeba, 
localization  is  found  not  only  in  the  function  of 
locomotion,  but  also  in  that  of  ingestion. 

In  a  relative  of  Paramecium,  Stylonychia  (see 
fig.  13),  the  cilia  are  themselves  differentiated  in 
various  regions  of  the  body  for  different  functions. 


^%ss^^. 


Fig.  13.  —  Stylonychia  creeping  on  its  oral  ("lower")  surface  viewed 
from  the  left  side  :  1 ,  anterior  cirri ;  2,  adoral  zone  of  membranelles  ; 
3,  anterior  branch  of  the  pulsating  vacuole  (4)  ;  ^,  dorsal  cilia  ;  6,  pos- 
terior branch ;  7,  caudal  cirri  ;  8,  posterior  cirri ;  9,  ventral  cirri.  — 
(From  Lang,  after  Biitschli  and  Schewiakoff.) 

Although  those  on  the  upper  side  have  almost  disap- 
peared, yet  Stylonychia  is  more  active  and  has  a 
greater  variety  of  movement  than  Paramecium. 
This  is  due  to  the  fact  that  the  cilia  on  the  lower  side 
of  the  cell-body  are  fused  together  in  places  to  form 
stout,  elastic,  hooklike  "  cirri,"  shaped  and  inserted 
in  such  a  variety  of  wa^  as  to  enable  Stylonychia 
to  crawl  and  jump.  Rows  of  ordinary  cilia  enable 
it  to  swim.  In  addition^^  these  forms  of  cilia,  there 
occurs  in  the  oral  groove  a/series  of  platelike  mem- 


36  GENERAL  BIOLOGY 

branes,  formed  by  the  fusion  of  many  cilia,  that  beat 
with  the  motion  of  a  fan  and  drive  the  food  down 
the  gullet  with  force  and  precision,  '^his  function 
is  further  subserved  by  undulating  membranes 
(see  fig.  13)  likewise  formed  of  fused  cilia.  Each  of 
these  types  of  locomotor  organs  has  a  special  function 
to  perform.  In  some  way  there  has  come  about  a 
division  oj  labor  among  the  cilia  in  different  parts  of 
the  body,  one  group  of  cilia  performing  one  function, 
another  group  another;  and  in  proportion  to  the 
extension  of  this  division  of  labor  there  has  arisen 
a  corresponding  efficiency  of  action  of  each  part. 
Along  with  this  physiological  specialization  of 
function  there  has  developed  the  corresponding 
modification  of  structure  which  we  have  called  dif- 
fer entiation.  Physiological  specialization  and  mor- 
phological differentiation  are  thus  two  connected 
consequences  that  result  from  a  division  of  labor 
among  the  parts  of  an  organism.  The  fundamental 
function  involved  in  the  examples  just  described 
is,  however,  in  each  case  the  contractility  of  the 
protoplasm.  It  is  in  the  method  or  the  physical 
basis  of  utilizing  this  function  that  efficiency  is 
attained ;  just  as  the  same  amount  of  current  from 
the  same  wire  will  produce  a  brighter  light  in  a 
tungsten  filament  than  in  a  carbon  incandescent 
lamp. 

The  limits  of  the  specialization  of  contractile 
organs  are  apparently  soon  reached.  In  some 
Protista,  however,  there  has  been  a  differentiation 
of    contractile    substance    within    the    cell-body,    a 


PRIMARY  FUNCTIONS  OF  THE  ORGANISM    37 


localization  of  the  function  of  contractility  to  certain 
regions  of  the  cell. 

In  Vorticella,  a  ciliate  protozoan  that  is  usually 
rooted  plantlike  to  a  fixed  base,  the  long  stalk  which 
the  cell-body  develops 
(see  fig.  14)  is  very 
contractile,  extending 
the  vorticella-bell  to 
a  considerable  dis- 
tance and  then  sud- 
denly pulling  it  back. 
During  this  move- 
ment the  stem  coils 
and  uncoils  like  a 
spring,  owing  to  the 
presence  in  it  of  a 
very  contractile  fiber 
of  differentiated  pro- 
toplasm. The  stem 
contains  little  else 
than  this  contractile 
fiber  (myoneme,  of 
authors) ,  and  we  may 
say  that  the  sole  func- 
tion of  the  stem,  aside 
from  that  of  support- 
ing the  "bell,"  is  to  withdraw  it  out  of  danger  or  ex- 
tend it  into  an  area  of  greater  food  supply.  In  return 
for  this  the  bell  eats  and  digests  and  reacts  for  both. 
Here  is  a  division  of  labor  that  has  proceeded  so  far 
that  a  different  kind  of  protoplasmic  substance  has 


Fig.  14.  —  Vorticella:  a,  extended; 
b,  contracted  ;  c,  the  stalk  more  highly 
magnified,  showing  the  contractile  fiber 
which  is  not  seen  in  a  and  b. 


38 


GENERAL   BIOLOGY 


been  segregated  from  the  rest,  and  in  this  area, 
one  primary  function  of  Hving  substance  has  been 
emphasized  to  the  practical  exclusion  of  the  others. 
If  a  cell-wall  should  form  across  the  base  of  the 
vorticella-bell,  and  if  both  cells  should  then  remain 
together,  we  should  be  justified  in  speaking  of  the 


Fig.  15.  —  Types  of  muscle-cells:  A  and  B,  smooth  muscle-cells;  C, 
two  fragments  of  cross-striated  muscle  fibers  (cells);  at  the  left  above, 
the  end  of  a  fiber.     Note  the  numerous  nuclei.  —  (Verworn.) 


contractile  member  of  this  two-celled  organism  as  a 
muscle-cell.  We  know  of  no  instance  of  such  a 
development  in  Vorticella  or  any  other  protozoan, 
but  in  the  Metazoa  the  segregation  of  the  contractile 
function  of  protoplasm  into  a  special  area  and  the 
differentiation  of  this  area  into  contractile  muscle- 
cells  is  almost  universal.  Such  a  cell  is  in  turn 
differentiated  with  respect  to  the  nature  of  the  con- 


PRIMARY  FUNCTIONS  OF  THE  ORGANISM    39 

traction  which  it  is  of  advantage  to  the  organism  to 
have  produced.  In  the  vertebrates  the  tissues  that 
carry  out  slow,  rhythmic  contractions,  such  as  those 
of  the  intestinal  walls,  are  made  up  of  small,  narrow, 
spindle-shaped  cells  (fig.  15)  with  a  single  nucleus 
and  a  delicate  longitudinal  striation.  The  skeletal 
muscles  and  those  throughout  the  animal  series  in 
which  rapid  or  "  voluntary  "  movement  is  produced 
are  very  highly  differentiated  (fig.  15  c).  In  the 
specialization  of  their  substance  along  the  line  of 
contractility  they  have  lost  the  function  of  food- 
taking,  and  to  a  great  degree,  though  not  entirely, 
that  of  conduction  and  general  irritability. 

Specialization  in  Conducting  Organs.  —  A  par- 
ticular kind  of  irritability,  however,  has  come  into 
play  in  connection  with  another  sort  of  cells  called 
nerves.  Nerve-cells  represent  another  line  of  spe- 
cialization. Here  the  function  of  specific  irritability 
and  conduction  has  been  developed,  until,  in  com- 
pensation, the  functions  of  contractility  and  nutri- 
tion have  entirely  disappeared. 

An  experiment  of  Professor  J.  Loeb's  demon- 
strates in  an  interesting  way  the  performance  of  the 
same  function  by  a  highly  specialized  tissue  and  by 
a  less  specialized  one.  One  of  the  group  of  degen- 
erate animals  called  Tunicates  is  provided  with 
two  siphons  (fig.  16)  or  passages,  through  one  of 
which  the  water  passes  into  the  saclike  body  and 
through  the  other  of  which  it  flows  out.  Midway 
between  the  two  is  situated  the  nervous  system. 


40 


GENERAL  BIOLOGY 


consisting  of  a  single  large  "  ganglion  "  or  nerve-knot, 
from  which  ramify  nerves  in  all  directions.  In 
Ciona,  a  member  of  this  group,  if  one  of  these  siphons 
is  touched  with  a  needle,  both  of  them  contract 
almost  simultaneously.     If,   however,   the  ganglion 

be  snipped  out  with 
a  pair  of  scissors  and 
one  of  the  siphons 
be  touched  with  a 
needle,  the  one  stim- 
ulated will  contract 
at  once,  but  only 
after  a  considerable 
interval  does  the 
other  siphon  like- 
wise contract.  There 
has  been  a  conduc- 
tion of  the  stimulus 
in  both  cases.  In 
the  first  instance  the 
nervous  system 
afforded  so  perfect 
a  means  of  conduc- 
tion that  the  contrac- 
tion of  both  siphons 
occurred  almost  together.  In  the  second  experi- 
ment, however,  the  only  path  of  conduction  lay 
through  the  intervening  muscle  fibers,  specialized 
along  the  line  of  contractility  but  not  that  of  con- 
duction. Hence  the  conduction  was  very  imper- 
fectly and  slowly  carried  out. 


c- 


Fig.  16.  —  Ciona  intestinalis,  a  Tuni- 
cate :  a  and  6,  the  two  siphons  ;  c,  foot ; 
d,  location  of  the  ganglion.  —  (From 
Loeb.) 


PRIMARY  FUNCTIONS  OF  THE  ORGANISM    41 

Secretion.  —  The  majority  of  cells  retain  in  greater 
or  less  degree  the  primary  function  of  protoplasm 
called  secretion.  By  virtue  of  its  chemical  and 
physical  organization  living  matter  not  only  builds 
non-living  food  substance  into  itself  to  form  new 
protoplasm  (nutrition),  but  it  reverses  the  process 
and  forms  from  its  living  substance  non-living  prod- 
ucts or  secretions.  Certain  types  of  cells  are  spe- 
cialized in  this  direction  to  a  great  degree,  and  we  speak 
of  them  as  secretory  or  gland  cells.  But  many  animal 
cells  and  nearly  all  plant  cells  secrete  a  denser  sub- 
stance about  themselves,  the  cell- wall,  and  further, 
a  cement  substance  which  binds  the  cells  together  into 
a  unified  mass.  Cellular  secretions  are  thus  of  two 
sorts,  "  permanent  "  secretions  that  remain  in  place 
after  being  formed,  and  secretions  which,  when 
elaborated,  are  passed  out  of  the  cell-body  to  other 
parts  of  the  organism.  In  plant  tissues  the  cell- wall 
is  usually  greatly  thickened  and  strengthened  by  the 
secretion  of  cellulose,  a  derivative  of  which  gives 
wood  its  hard  quality.  In  animal  tissues  the  cell- 
wall  is  not  usually  so  thickened,  but  a  similar  result 
is  obtained  by  the  development  of  intercellular 
substances.  These  give  the  connective  tissues  their 
characteristic  structure  and  qualities.  In  the  latter 
the  cells  are  specialized  in  the  production  of  secre- 
tions until  the  functions  of  contractility,  irritability, 
and  conduction  have  almost  if  not  entirely  dis- 
appeared, and  the  intercellular  substance  is  produced 
in  such  quantities  as  to  outbulk  many  times  the 
living   cells   themselves.     According   to   the   nature 


4^  GENERAL  BIOLOGY 

of  this  intercellular  substance  the  connective  tissue 
is  described  as  white  fibrous  tissue,  yellow  elastic 
tissue,  cartilage,  bone,  etc.  Strikingly  dissimilar 
as  these  types  of  tissue  are,  they  not  only  are  ex- 
amples of  the  same  sort  of  protoplasmic  activity, 
but  all  of  them  in  development  have  been  shown  to 
be  derived  from  an  original,  much  more  generalized 
type  of  primitive  connective  tissue  cell. 

In  the  other  phase  of  secretory  activity  mentioned, 
the  production  of  secretions  which  are  freely  trans- 
ported to  other  parts  of  the  organism,  we  have  a 
very  easily  apprehended  example  of  the  division  of 
labor  between  the  parts. 

Specialization  in  Digestion.  —  Amoeba,  when  it  has 
ingested  a  particle  of  food-substance,  digests  it  by. 
dissolving  it  so  that  it  can  be  assimilated  into  the 
protoplasm.  It  does  so  by  means  of  minute  quan- 
tities of  a  substance  which  it  secretes  that  acts 
chemically  upon  the  food.  (See  Chapter  III.)  This 
process  of  digestion,  like  that  of  ingestion,  may  take 
place  in  all  parts  of  the  cell-body,  i.e.  it  is  not  localized. 
In  the  complex  structures  composing  the  body  of  a 
higher  animal  it  is  obvious  that  it  would  be  impossible 
for  food  to  come  in  contact  with  every  part  of  the 
body  so  that  each  cell  could  attend  to  its  own 
function  of  digestion.  Accordingly,  we  find  that 
the  principle  of  the  physiological  division  of  labor 
comes  into  play  in  very  simple  and  otherwise  quite 
generalized  forms  (such  as  Hydra).  The  secretion 
of  digestive  fluids  and  the  function  of  alimentation 


PRIMARY   FUNCTIONS   OF  THE   ORGANISM    43 

in  general  becomes  restricted  first  to  a  layer  of  cells 
(in  Hydra)  and  then  to  localized  regions  of  this 
layer  (digestive  portion  of  the  alimentary  canal), 
the  specific  secretions  for  different  kinds  of  food 
eventually  being  segregated  in  higher  animals  in 
different   areas   of   the  canal    (stomach,    duodenum. 


Fig.  17.  —  Transverse  section  of  Hydra,  showing  the  coelenteric  cavity 
and  the  two  layers  of  the  body  wall.  —  (Shipley  and  McBride.) 

etc.).  Such  an  adaptation  makes  for  much  greater 
efficiency.  However,  in  some  parasitic  worms,  such 
as  the  tapeworm,  which  lies  in  the  alimentary  canal 
of  its  host  and,  so  to  speak,  does  not  have  to  exert 
itself  to  get  or  digest  food,  the  localized  apparatus 
for  alimentation  has  disappeared,  and  the  digested 
food  supplied  by  the  host  is  absorbed  directly  through 
the  body- wall  of  the  worm. 

In  connection  with  the  specific  character  of  the 


44 


GENERAL  BIOLOGY 


secretions  of  gland  cells  has  arisen  the  necessity  for 
the  production  of  large  quantities  of  the  secretion  at 
one  time  in  a  limited  space.  The  natural  extension 
of  the  secreting  area  is  ordinarily  quickly  limited 
by  the  need  for  concentrating  the  secretion  at  a 
certain  point.  In  consequence  the  secretory  surface 
is  increased  by  folding  and  by  sinking  below  the 


Fig.  18.  —  Diagram  of  the  formation  of  glands  by  the  sinking  in  of  a 
secretory  area  of  the  epithelium:  1,  simple  tubular  gland;  2  and  3, 
branched  tubular  glands  ;  4-  and  5,  simple  alveolar  glands  ;  6,  branched 
alveolar  gland.  —  (Hertwig.) 


surface  of  the  cell-layer  (epithelium)  of  which  it  is  a 
part,  so  that  a  maximum  amount  of  secreting  surface 
is  produced  in  a  minimum  of  space.  In  this  way  is 
developed  a  glmid  (see  fig.  18),  the  common  channel 
for  the  substance  secreted  being  the  duct. 

All  organisms  possess  the  power  of  reproducing 
themselves,  otherwise  the  species  of  which  they  are 
members  would  become  extinct.  In  the  higher 
plants  and  animals    this,  like  the  other   properties 


PRIMARY  FUNCTIONS  OF  THE  ORGANISM    45 

of  living  matter,  has  become  the  special  function 
of  a  certain  class  of  cells,  the  so-called  germ-cells.  In 
these  cells  as  in  other  classes  we  can  trace  the 
gradual  increase  of  efficiency  and  certainly  of  action 
of  the  function  through  a  division  of  labor. 

Summary.  —  We  have  seen  that  the  organism, 
whether  simple  or  complex,  may  be  looked  upon  as 
a  machine  that  does  certain  things ;  in  other  words, 
"  works."  The  nature  of  the  work  that  the  protoplas- 
mic machine  does  sets  it  off  from  all  others  as  some- 
thing unique  and  (at  present  at  least)  inimitable.  No 
machine  that  man  has  ever  made  reproduces  itself, 
repairs  itself,  or  automatically  adjusts  itself  to  chang- 
ing external  conditions.  These  primary  functions, 
however,  are  carried  out  with  varying  degrees  of 
perfection  by  different  sorts  of  organisms.  As  a 
rule  the  organisms  of  comparatively  complex  struc- 
ture perform  these  functions  as  a  unit  more  efficiently 
than  those  of  a  lower  grade  of  organization.  This 
is  due  to  the  fact,  as  we  have  seen,  that  the  functions 
are  performed  by  various  parts  of  the  whole  organism 
particularly  adapted  for  the  purpose.  Physiological 
specialization  and  structural  differentiation  come 
into  play  side  by  side  as  the  result  of  a  division  of 
labor.  An  organism  in  which  this  phenomenon 
has  been  extensively  developed  is  spoken  of  as 
specialized.  One,  such  as  Amoeba,  in  which  there  is 
little  or  no  specialization,  is  called  generalized.  As 
a  rule  this  is  the  criterion  by  which  we  estimate  the 
position  of  a  plant  or  animal  in  the  scale  of  life. 


46 


GENERAL  BIOLOGY 


A  "  low  "  form  is  a  generalized  one,  a  "  higher  " 
form  is  one  that  is  specialized.  This  "  criterion  of 
perfection  "  confronts  the  student  of  nature  at 
every  turn,  but  the  method  by  which  such  a  condition 
comes  about  is  the  great  central  problem  of  biology 

as  yet  unsolved. 
Specialization  af- 
fects particularly 
the  cells,  since  it 
is  these  that  are 
structurally  dif- 
ferentiated. In 
proportion,  how- 
ever, to  the  degree 
of  their  specializa- 
tion they  lose  their 
own  independent 
self-sufficient  iden- 
tity   and    become 

Fig.  19.  —  Diagrams  of  wings,  showing  m  C  r  ff  e  d  W  i  t  ll 
homology    and    analogy :    a,  wing   of  a  fly  ;  , 

h,  wing  of  a  bird  ;    c,  wing  of  a  bat.     c  is  the  Otlicrs       lUtO       ag- 

homologue  of  h,  «  is  an  analogue. —  (Jordan  o-r^o-at^G      rkf      i^f^llc 

and  Kellogg.)  gregates    or    cells 

of  similar  func- 
tion. These  aggregates  we  have  already  referred 
to  as  tissues. 

Again,  for  the  better  carrying  out  of  the  various 
functions  of  the  whole  organism,  different  tissues 
are  combined  into  organs.  The  stomach,  which 
not  only  digests  food,  but  also  kneads  it  and  breaks 
it  up,  is  made  up,  in  addition  to  secretory  tissue,  of 
muscular   and   connective   tissues   as   well.     Again, 


PRIMARY  FUNCTIONS  OF  THE  ORGANISM    47 

organs  of  allied  and  connected  functions  are  com- 
bined into  systems,  some  of  which,  like  the  nervous 
and  circulatory  systems,  pervade  the  whole  organism. 
It  is  of  great  interest  and  value  to  compare  animals 
and  plants  with  respect  to  the  degree  of  specializa- 
tion of  their  parts ;  for  such  a  comparison  often  re- 
veals relationships.  In  making  such  comparisons  it  is 
sometimes  found  that  organs  carrying  out  the  same 
function,  such  as  the  wing  of  a  bird  and  that  of  a 
butterfly,  are  of  a  very  diverse  origin  and  structure. 
On  the  other  hand,  the  wing  of  a  bird  and  the  foreleg 
of  a  dog,  in  spite  of  the  apparently  very  different 
functions  which  each  performs,  have  each  the  same 
origin  relative  to  the  rest  of  the  body  and  the  same 
general  internal  structure.  Such  a  similarity  we 
call  homology,  and  we  speak  of  the  two  parts  as 
homologous,  whereas  the  similarity  of  function  be- 
tween the  wing  of  a  bird  and  of  a  butterfly  we  speak 
of  as  analogy,  and  the  parts  as  analogous. 


CHAPTER  III 

METABOLISM 

Oxidation.  —  It  is  known  that  oxygen  exists  in 
two  forms,  ordinary  atmospheric  oxygen  and  ozone, 
the  molecule  of  the  former  consisting  of  two  atoms 
(written  O2),  and  that  of  the  other  of  three  (O3). 
During  thunderstorms  ozone  is  often  formed  from 
oxygen  by  condensation,  through  the  action  of 
electricity.  Both  gases  are  chemically  active  in 
combining  with  other  elements  or  compounds,  a 
process  known  as  oxidation.  The  activity  of  ozone, 
however,  is  much  greater  than  that  of  ordinary 
oxygen.  It  gives  up  its  extra  atom  of  O  with  facility, 
and- is,  therefore,  spoken  of  as  less  stab4e  than  the 
latter.  But  the  resulting  product  of  oxidation  by 
either  oxygen  or  ozone  is  *  exactly  the  same.  The 
only  difference  between  the  two  must  be  the  way 
in  which  the  atoms  of  O  are  combined  to  form  the 
molecule.  In  oxidation  there  is  an  evolution  of 
heat,  which  is  the  release  of  the  intrinsic  energy  of 
combination  in  the  oxygen  or  ozone  molecule. 
Measurements  have  shown  that  in  the  oxidation  of 
finely  divided  platinum  by  ozone,  some  72,400 
calories  ^  more  of  heat  per  gram  is  evolved  than  in 
the  corresponding  oxidation  by  ordinary  oxygen. 
This  figure  must  represent  the  additional  amount  of 

1  A  calorie  is  the  measure  of  heat  required  to  raise  one  cubic  centimeter 
of  water  one  degree. 

48 


METABOLISM  49 

energy  locked  up  in  the  ozone  molecule  in  comparison 
with  the  oxygen  molecule.  When  we  speak  of  this 
energy  as  being  locked  up  or  "  latent,"  we  have 
exhausted  our  knowledge  of  it.  We  know  that  it  is 
there,  holding  the  atoms  together  by  what  we  call 
"  chemical  affinity,"  and  that,  when  the  atoms  are 
released  from  their  bonds,  this  energy  becomes 
evident  to  our  senses ;  that  is,  it  becomes  kinetic, 
and  assumes  various  forms,  such  as  heat,  light,  elec- 
tricity, or  motion. 

Two  points  of  great  significance  must  be  noted  in 
the  illustration  just  given ;  first,  the  fact  that  the 
more  complex  substance  (O3)  has  latent  a  great 
deal  more  of  the  energy  of  combination  than  the 
simpler  one  (O2) ;  and  secondly,  that  the  former 
breaks  down  or  gives  up  its  latent  energy  more  readily 
than  the  latter.  Few  atomic  combinations  are  as 
simple  as  oxygen  and  ozone  and  at  the  same  time 
are  so  readily  disrupted ;  indeed,  among  the  more 
complex  substances  it  is  a  general  rule  that  the  greater 
the  complexity  of  the  molecule,  the  greater  the 
amount  of  its  potential  energy  and  the  greater  its 
instability. 

The  storing  up  of  the  potential  energy  of  "  chem- 
ical affinity  "  is  well  illustrated  by  a  so-called  endo- 
thermic  compound  such  as  acetylene,  —  C2H2 : 

Heat  of  combustion  of  C2  =  2  X  96,980  cal.  =  193,960  cal. 

Heat  of  combustion  of  H2  =  2  X  34,960  cal.  =    69,920  cal. 

Total  263,880  cal. 

Heat  of  combustion  of  C2H2  (acetylene)     .      =  310,600  cal. 

Difference         .  46,720  cal. 


50  GENERAL   BIOLOGY 

That  is,  the  energy  that  binds  together  the  hydro- 
gen and  carbon  in  the  form  of  acetylene  is  nearly 
one  sixth  greater  than  the  sum  total  of  the  intrinsic 
energy,  measured  by  the  heat  evolved  in  combus- 
tion, of  the  same  amount  of  either  element  taken 
separately.^ 

Conservation  of  Energy.  —  In  the  above  example 
the  potential  energy  is  released  as  heat,  but  it  is 
conceivable,  on  the  basis  of  other  experiments,  that 
this  energy,  released  by  the  breaking  down  of  acet- 
ylene, might  be  employed  at  once  in  building  up 
some  other  chemical  compound,  and  thus  be  non- 
evident  to  us  except  from  its  end-result.  At  any 
rate  the  decomposition  and  recombination  (analysis 
and  synthesis)  of  any  chemical  compound  may  be 
repeated  indefinitely,  the  same  amount  of  energy 
being  released  or  absorbed  at  each  change.  All  the 
forms  of  energy  known  to  us  may  be  transformed  one 
into  another  in  this  way.  Gravitation  acting  on  the 
molecules  of  water  in  a  brook  may  be  caused  to  drive 
a  water  wheel.  The  wheel,  by  its  motion,  in  addition 
to  producing  heat  by  friction,  may  also  run  a  dynamo, 

1  The  above  figures  refer  to  the  heat  of  combustion  of  one  gram  each 
of  carbon,  hydrogen,  and  acetylene.  If  equivalent  amounts  of  sub- 
stance be  taken  the  difference  is  even  more  striking.  The  molecular 
weight  of  acetylene  is  26  and  every  gram  of  acetylene  contains  twenty- 
four  twenty-sixths  of  a  gram  of  carbon  (mol.  wt.  of  C  =  12)  and  two 
twenty-sixths  of  a  gram  of  hydrogen  (H  =  1) ;  twenty-four  twenty- 
sixths  of  193,960  cal.  is  179,040  cal.  and  two  twenty-sixths  of  69,960  cat 
is  5,378  cal,;  the  sum  of  these  is  184,418  cal.  or  126,182  cal.  less  than 
the  heat  of  combustion  of  one  gram  of  acetylene. 


METABOLISM  51 

the  electricity  generated  by  which  affords  us  light, 
heat,  and  such  chemical  analyses  and  syntheses  as 
are  involved  in  cooking.  In  all  these  transformations 
of  one  kind  of  energy  into  another,  though  much  is 
wasted  in  manipulation,  none  is  lost ;  in  other  words 
all  can  be  accounted  for.  This  conception,  that 
the  sum  of  energy  in  the  universe  is  constant  and 
merely  changes  its  form  from  one  kind  to  another, 
is  one  of  the  great  generalizations  of  natural  science, 
and  is  known  as  the  "  Law  of  the  Conservation  of 
Energy." 

Chemistry  teaches  us  also  that  the  more  complex 
"  organic "  compounds  are  built  up  of  relatively 
few  kinds  of  atoms ;  but  these  are  combined  and 
recombined  into  groups  of  higher  and  higher  orders, 
an  absorption  of  energy  taking  place  with  every 
combination,  until  a  huge  aggregate  results,  the 
potential  energy  of  which  is  enormous.  Thus,  a  mole- 
cule of  a  simple  sugar,  dextrose  (C6H12O6),  may  be 
combined  with  another  molecule  of  a  similar  sugar 
to  form  a  new  sugar  of  a  higher  order,  cane-sugar 
or  sucrose  (Ci2H220ii).^  In  this  reaction  a  mole- 
cule of  water  is  subtracted.  More  molecules  may  be 
added  to  the  combination  over  and  over  again,  like 
keys  on  a  keyring,  if  the  molecule  of  water  is  each 
time  removed.  With  every  such  combination  there 
is  an  addition  to  the  amount  of  potential  energy 
accumulated  in  the  molecule,  and  of  course  this 
energy  must  be  supplied  from  without.  Usually 
its  source  is  the  heat  of  the  alcohol  or  gas  flame  which 

1  C6H12O6  +  CeHiaOe  =  C12H22O11  +  H2O. 


52  GENERAL  BIOLOGY 

the  chemist  supplies.  Furthermore,  if  a  compound 
sugar,  Kke  the  cane-sugar  just  described,  should 
be  broken  up,  it  would  not  resolve  itself  into  its 
ultimate  components  (atoms)  at  once,  but  the  line 
of  cleavage  would  occur  first  at  the  point  where  the 
two  larger  groups  had  joined.  In  other  words,  the 
affinity  of  the  two  simple  sugars  for  one  another  is 
much  weaker  than  the  affinities  of  their  constituent 
atoms  for  each  other.  In  general,  the  simplest 
compounds,  such  as  CO2,  H2O,  NH3,  etc.,  are  bound 
together  by  very  strong  chemical  affinity  and  require 
much  force  to  disrupt  them,  whereas  the  chemical 
affinity  binding  together  very  complex  organic 
substances  is  usually  so  weak  that  these  molecules 
often  appear  to  disintegrate  spontaneously,  for 
which  reason  they  are  spoken  of  as  unstable.  These 
facts,  as  we  shall  see,  have  an  important  bearing  on 
the  utilization  of  food  substances  by  plants  and 
animals. 

Chemical  Synthesis  in  the  Organism.  —  Green 
plants  not  only  require  the  normal  conditions  of 
heat  and  moisture  demanded  by  all  living  things, 
but  they  require  sunlight  as  well,  else  they  grow 
pale  and  sickly.  This  is  true  even  of  those  plants, 
like  ferns,  that  thrive  best  in  the  shade.  The 
position  and  attitude  of  every  leaf  on  a  tree  is  ad- 
justed to  receive  the  maximum  amount  of  sunshine. 
If  we  inclose  a  leaf  in  a  glass  tube  filled  with  CO2, 
and  then  expose  it  to  the  sunshine,  after  several 
hours  we  will  find  by  tests  that  a  large  part  or  all 


METABOLISM 


53 


of  the  CO2  has  disappeared  and  has  been  replaced 
by  an  equal  volume  of  oxygen.  The  total  volume 
of  the  gas  has  not  been  altered,  but  we  find  that 
the  carbon  has  disappeared  from  the  tube.  And  since 
this  change  will  not  take  place  in  the  dark,  even 
if  the  other  conditions  be  similar,  we  conclude  that 
the  sunlight  has 
supplied  the  neces- 
sary energy.  What, 
then,  has  become 
of  the  carbon  ? 

Green  plants  al- 
ways have  a  certain 
amount  of  food  ma- 
terial stored  up  in 
the  leaves,  usually 
in  the  form  of  starch. 
The  presence  of 
starch  may  be  easily 
detected  by  testing 
with  a  solution  of 
iodine,  which  colors 
it    a   bright  blue. 


Fig.  20.  —  Experiment  showing  the 
function  of  sunlight  in  the  synthesis  of 
starch  in  the  green  leaf.  A  slice  of  cork  is 
fastened  over  the  leaf  and  the  rest  of  the 
leaf  exposed  to  the  sunlight.  The  figure 
to  the  right  shows  the  result  when  the  cork 
is  removed  and  the  leaf  dipped  in  solution 
of  iodine.  —  (Bailey  and  Coleman.) 


m 


If  we  keep  such  a  plant 
the  dark  for  a  while,  it  will  exhaust  this  store 
of  starch,  as  may  be  shown  by  the  leaves  giving  a 
negative  test  with  the  iodine.^  If,  however,  we 
pin  a  strip  of  cork  across  part  of  a  leaf  from  which 
the    starch    has    been    exhausted,    and    expose    the 

1  In  such  an  experiment  the  chlorophyll  must  be  dissolved  out  in  hot 
alcohol  before  the  iodine  is  applied,  in  order  that  its  green  color  may  not 
mask  the  starch  reaction. 


54  GENERAL   BIOLOGY 

partially  covered  leaf  to  the  sunshine  for  a  while, 
we  find,  when  we  treat  it  with  iodine,  that,  whereas 
the  strip  covered  by  cork  gives  a  negative  test  as 
before,  the  rest  of  the  leaf  which  was  exposed  to  the 
sunshine  turns  a  deep  blue  with  the  iodine.  This 
demonstrates  that  starch  has  been  actually  formed 
in  that  part  of  the  leaf  which  has  received  the  sun's 
rays.  This  conclusion  we  can  confirm  with  the 
microscope,  for  the  starch  granules  may  be  found 
in  the  chlorophyll  bodies  of  the  cells  after  such  ex- 
posure to  sunlight. 

Out  of  the  water  absorbed  by  the  plant,  and  the 
CO2  always  present  in  the  air,  in  the  presence  of 
chlorophyll  and  the  sunshine,  the  plant  can  synthe- 
size starch,^  which  may  be  conveyed  (in  the  form  of 
sugar)  to  other  parts  of  the  plant-body  to  be  stored 
up  as  reserve  food  or  utilized  at  once  as  a  source  of 
energy  for  the  vital  processes  of  the  plant.  The 
CO2  in  the  air  is  the  source  of  the  bulk  of  the  carbon 
compounds  in  the  substance  of  the  plant.  This  is 
shown  by  the  fact  that  most  plants  grown  in  an 
atmosphere  free  from  CO2  die  (of  starvation)  even 
if  the  soil  in  which  they  are  rooted  be  richly  supplied 
with  carbon  compounds.  Conversely,  they  will 
thrive  in  a  substratum  free  from  carbon  compounds 
if  they  have  access  to  ordinary  air. 

Although  starch  is  the  first  evident  product  of 
this    process    (photosynthesis),   yet    it  is    probably 

1  6  CO2  +  6  H2O  =  (CeHioOe)  +  6  O2  ...  (CeHiaOe)^  (-  n  H2O 
glucose 

=  (CeHioO,,)^. 
starch 


METABOLISM      '  55 

only  the  end-product  of  a  long  series  of  changes. 
The  carbohydrate  food  in  many  plants  is  more  often 
in  the  form  of  a  dilute  solution  of  sugar,  which,  of 
course,  is  much  less  easily  demonstrated  than  the 
solid  starch. 

Production  of  Fats  and  Proteins.  —  We  know  of  no 
other  food  than  the  starch  that  is  synthesized  from 
such  simple  chemical  compounds,^  and  it  has  been 
shown  that  the  fats  and  proteins  are  produced  from 
the  starches  as  a  foundation.  The  spontaneous  pro- 
duction of  fatty  oils  in  seeds  containing  starch  has 
been  directly  observed,  and,  since  the  fats  contain 
no  elements  not  also  found  in  the  carbohydrates, 
such  transformation  involves  no  more  than  a  re- 
arrangement of  these  elements  in  the  molecule.  It  is 
known  that  water  and  CO2  are  produced  as  the  result 
of  such  a  change,  although  we  have  still  much  to 
learn  of  the  intermediate  steps  in  the  process. 

The  explanation  of  the  origin  of  the  proteins  is  much 
more  difficult,  for  proteins  possess,  in  addition  to  the 
carbon,  hydrogen,  and  oxygen  of  starch,  a  high  percent- 
age of  nitrogen,  as  well  as  sulphur,  and  often  phos- 
phorus. These  latter  elements  are  obtained  from  the 
soil  in  the  form  of  nitrates  (or  nitrites),  sulphates,  and 
phosphates,  and  are  absorbed  through  the  roots  of  the 
plant  in  solution.  The  nitrates  (sodium  or  potassium) 
probably  enter  into  union  with  the  carbohydrate  radicle 
to  produce  some  simple  amino-acid  such  as  asparagin 
(C4H8N2O3).     By  a  succession  of  syntheses  involving 

^  Other  organisms  recently  discovered  can  apparently  utilize  carbon 
monoxide-  (coal  gas),  while  still  others  may  use  methane  (marsh  gas)  as 
their  only  source  of  carbon. 


56  GENERAL  BIOLOGY 

the  condensation  of  these  amino-acids,  other  radicles 
or  "albuminous  nuclei"  are  welded  on,  combined,  and 
recombined  with  the  carbohydrate  base,  or  with  each 
other,  each  time  with  the  disappearance  of  the  poten- 
tial energy  of  chemical  affinity,  until  the  huge,  com- 
plex, unwieldy,  protein  molecule  results,  and  becomes 
a  part  of  the  mixture  we  call  protoplasm.  This  pro- 
cess has  been  shown  to  take  place  in  the  green  leaves, 
although  it  goes  on  in  the  dark. 

To  summarize :  The  plant  in  its  myriads  of  cell- 
laboratories  is  constantly  carrying  on  a  variety  of 
chemical  syntheses.  First,  the  carbon  element  of 
the  CO2  derived  from  the  air,  and  the  H2O  absorbed 
from  the  soil,  are  combined  in  the  green  leaves  to 
form  carbohydrates,  such  as  sugars  and  starch. 
These  molecules  may  be  welded  together  to  form 
more  complex  carbohydrates,  such  as  dextrine, 
cellulose^  or  wood  fiber,  or  they  may  be  combined 
and  recombined  with  other  elementary  compounds 
containing  nitrogen  and  sulphli^  tintil'  the'  complex 
and  unstable  albuminous  or  protein  molecule  results. 
Each  step  involves  the  change  of  kinetic  energy 
into  potential  energy,  and  all  this  energy  is  derived 
in  the  first  instance  from  sunlight.  This  building-up 
process  from  simple  to  complex  is  called  Anabolisniy 
and,  as  a  consequence  of  this  process,  the  plant  is 
endowed  with  an  immense  reserve  of  potential 
energy. 

Dissimilation.  —  But  the  plant  is  all  the  while 
living,  —  developing  new  buds  and  leaves,  maturing 
its    fruit,    secreting    characteristic    products,    even 


METABOLISM  57 

moving,  although  its  movements  may  not  always 
be  evident.  Says  Huxley,  referring  to  the  constant 
streaming  and  circulation  of  the  protoplasm  in  the 
cell :  "  The  wonderful  noonday  silence  of  a  tropical 
forest  is  after  all  due  only  to  the  dullness  of  our 
hearing;  and  could  our  ears  catch  the  murmur  of 
these  tiny  maelstroms  as  they  whirl  in  the  innumer- 
able myriads  of  living  cells  which  constitute  each  tree 
we  should  be  stunned  as  with  the  roar  of  a  great  city." 
All  these  phenomena,  as  we  know,  are  but  mani- 
festations of  energy.  What  is  its  source  .^^  From 
careful  experiment  we  have  learned  that  it  is  not  only 
the  breaking  down  of  the  circulating  food  substances, 
but  may  be  the  disintegration  of  the  protoplasm 
itself.  Made  up  of  complex  aggregations  of  matter 
held  together  by  the  power  of  chemical  affinity,  the 
protoplasm  is  a  storehouse  of  potential  energy  that 
may  be  translated  into  kinetic  energy  by  the  disinte- 
gration of  the  unstable  compounds  composing  it.  In 
proportion,  then,  as  the  plant  does  work  of  any  sort,  it 
draws  on  its  own  substances  for  the  energy  requisite. 
Here  we  have  the  direct  reverse  of  the  building-up 
process  just  described.  The  circulating  food  sub- 
stances or  the  living  tissue  itself  is  constantly  break- 
ing down  and  as  constantly  being  renewed.  This 
continuous  flux  and  flow  is  called  Metabolism,  the 
tearing  down  process,  Kataholism. 

Metabolism  in  Animals. — The  whole  animal  world 
is  dependent  upon  the  plant  world  for  its  existence, 
since  even  the  flesh-eaters  depend  ultimately  upon 


58  GENERAL  BIOLOGY 

the  plant-eaters  for  food.  For  animals,  unlike  plants, 
are  quite  unable  to  utilize,  directly,  the  energy  of 
the  sun's  rays,  and  combine  into  sugars  and  starches 
the  water  and  CO2  with  which  they  are  surrounded. 
Nor  can  they,  like  plants,  utilize  the  nitrogen  as  it 
exists  in  simple  combination.  Nitrogen  is  an  es- 
sential element  of  protein,  and  protein  an  essential 
of  protoplasm,  and  without  it  the  animal  cannot 
repair  the  wastes  of  katabolism.  But  this  nitrogen 
must  be  furnished  to  the  animal  already  combined  in 
proteins. 

The  metabolism  of  animals  therefore  begins  at  a 
higher  level  than  that  of  plants.  Plants  take  in  and 
assimilate  gases  and  liquids  of  very  simple  composi- 
tion, whereas  animals  require  liquid  or  solid  food  al- 
ready organized  as  fats,  carbohydrates,  or  proteins. 
The  latter  kind  of  nutrition  is  sometimes  referred  to 
as  Holozoic,  the  former,  which  is  characteristic  of  all 
green  (chlorophyll-bearing)  plants,  as   Holophytic. 

Some  groups  of  plants,  however,  show  decided 
exceptions  to  such  a  rule.  The  bacteria  and  the 
fungi,  for  instance,  lack  chlorophyll  and  cannot 
manufacture  starch,  but  must  depend  on  other  or- 
ganisms, either  plant.or  animal,  for  their  food  supply. 
Such  plants  are  called  parasitic  when  they  feed  on 
living  tissue,  or  saprophytic  when  they  subsist  on  dead 
and  decaying  tissue.  Even  some  of  the  higher 
plants,  such  as  the  dodder,  a  relative  of  the  morning 
glory,  have  abandoned  the  independent  manufacture 
of  their  own  food  materials  and  live  as  parasites 
on  other  plants. 


METABOLISM 


59 


Fig.  21.  —  Leaf  of  the  Venus'  fly-trap  :  A,  open  ;  B,  closed, 
three  sensitive  hairs  on  each  leaflet  of  A. 


Note  the 


Some  plants,  indeed,  might  be  called,  if  not  holo- 
zoic,  at  least  "  amphizoic,"  since  they  have  de- 
veloped means  of  catching  and  killing  living  animal 
prey.  Of  such  are  the  familiar  "  Venus'  Fly  Trap  " 
(Drosera),    or    the    "Pitcher    plant"    (Nepenthes), 


Fig,  22.  —  Two  leaves  of  the  sundew  (Drosera  rotundifolia)  ;  the 
one  to  the  right  in  the  expanded  condition,  that  to  the  left  shortly  after 
the  capture  of  an  insect ;  the  tentacles  of  the  right  half  are  bent  over  to 
bring  the  glandular  tips  in  contact  with  the  prey.  Magnified  2|  times. 
—  (From  Barnes,  after  Kerner.) 


60  GENERAL  BIOLOGY 

etc.  In  the  ingenious  traps  possessed  by  such 
plants  unwary  insects  are  caught  and  killed ; 
digestive  fluids  there  secreted  dissolve  the  tissues 
of  the  prey,  and  they  are  absorbed  precisely  as 
they  would  be  in  the  stomach  of  a  carnivorous 
animal. 

Foods  in  General.  —  In  the  light  of  what  has  just 
been  said,  it  will  be  seen  that  we  must  modify  our 
notions  of  foods.  It  is  not  enough  to  classify  foods 
as  the  scientific  cook  books  do,  —  merely  as  carbo- 
hydrates, fats,  and  proteins.  Since  the  sole  purpose* 
of  taking  food  is,  as  we  have  seen,  the  accumulation 
of  a  store  of  energy,  we  might  define  a  food  to  be 
anything  that  contains  potential  energy.  The  CO2  and 
H2O  are  foods  to  a  green  plant  only  when  combined 
with  the  energy  of  sunlight.  They  are  better  called 
food-materials.  A  welsh  rarebit,  the  food  value  of 
which  is  very  high,  which  I  may  eat  with  impunity, 
may  be  "the  other  man's  poison."  But  a  stick  of 
hickory  that  supplies  the  wood-boring  beetle  larva  all 
the  nourishment  it  requires,  is  to  me  useless  because, 
for  my  purposes,  the  large  amount  of  energy  locked 
up  in  it  is  not  available.  If,  however,  I  reduce  the 
stick  to  sawdust  and  boil  it  with  sulphuric  acid, 
thereby  converting  it  into  glucose,  it  becomes  a  very 
good  food.  We  must,  therefore,  modify  our  previous 
definition  by  designating  as  a  food  anything  that 
contains  available  potential  energy. 

Fate  of  the  Foods  in  the  Higher  Animals.  —  In 


METABOLISM  61 

animals  the  fats  and  carbohydrates  yield  a  ready 
source  of  energy  in  the  form  of  heat.  Whether  they 
are  always  directly  oxidized  in  the  animal  body  with- 
out ever  having  become  part  of  the  tissue  itself  is 
perhaps  questionable,  but  this  seems  to  be  true  of 
the  carbohydrates  if  not  of  the  fats.  The  liver 
functions  in  an  important  way  in  the  carbohydrate 
metabolism  of  Vertebrates.  The  digested  sugar  is 
transformed  into  another  carbohydrate  called  gly- 
cogen ("  animal  starch  "),  and  stored  up  in  the  liver, 
and  later  in  the  muscles,  in  the  form  of  granules. 
The  glycogen  is  dissolved  and  given  back  to  the  blood 
stream  as  the  body  requires  it  between  meals,  or  is 
oxidized  in  place,  to  release  the  energy  involved  in 
muscular  work. 

Similarly,  the  fats  are  stored  up  in  the  different 
parts  of  the  body  or  in  special  organs  (the  fat-body 
of  the  frog,  e.g.)  to  be  drawn  on  as  need  arises.  If 
neither  fats  nor  carbohydrates  are  available,  the 
proteins  in  the  blood  stream  or  even  those  of  the 
tissue  itself  may  be  broken  down  to  supply  the 
necessary  energy.  Hence,  the  fats  and  carbohydrates 
are  often  spoken  of  as  the  "  protein-sparing  "  sub- 
stances. 

In  the  digestive  tract  of  the  higher  animals  the  fats 
are  split  into  their  components,  glycerine  and  fatty 
acid,  through  the  action  of  the  enzyme,  lipase.^  Being 
absorbed  in  this  form  they  are  recombined  in  the  epi- 
thelial cells  or  within  the  capillaries  and  circulate  as 

1  See  page  83. 


62  GENERAL  BIOLOGY 

fats  in  the  blood,  or  are  laid  down  in  various  parts  of 
the  body,  unchanged.  The  carbohydrates  are  all  split 
into  simple  sugars  {e.g.  glucose)  before  being  absorbed, 
and  possibly  circulate  in  the  blood  loosely  combined 
with  the  serum-proteid  base  in  the  same  way  that 
oxygen  combines  with  haemoglobin. 

The  proteins  follow  a  more  complicated  path,  al- 
though our  knowledge  of  them  is  confined  almost 
wholly  to  what  we  know  of  the  metabolism  of  warm- 
blooded animals.  Using  the  digestive  fluids  of  the 
alimentary  canal,  we  can  split  up  protein  (a  strip  of  lean 
meat,  for  example)  into  smaller  and  smaller  bodies 
until  we  reach  the  amino-acids,  which  are  the  units  out 
of  which  the  proteid  molecule  is  built  up.  These  are  ab- 
sorbed through  the  blood-vessels  of  the  alimentary  canal 
and  are  apparently  split  further  into  urea,  CO(NH2)2, 
on  the  one  hand,  and  on  the  other  hand  a  residue 
which  is  then  resynthesized  into  complex  albumens 
that  circulate  in  the  blood  stream  or  are  built  up  into 
the  protoplasm  of  the  tissues.  Although  an  absolute 
essential  for  the  maintenance  of  life,  nitrogen  is  not  ac- 
cumulated in  the  body ;  the  greater  the  amount  of  ni- 
trogenous food  ingested,  the  greater  the  amount  of  urea 
eventually  excreted,  —  a  condition  known  as  nitrogenous 
equilibrium. 

Role  of  Oxygen  in  Metabolism.  —  It  is  a  matter  of 
familiar  experience  that  all  animals  require  sNrafiundaiit 
supply  of  oxygen  in  order  to  live.  The  air-breathing 
vertebrates  are  especially  sensitive  to  the  lack  of 
this  element,  and  if  deprj^ed^of  a  supply  of  oxygen 
soon  succumb  with  characteristic  symptoms  of 
asphyxiation.  If  we  boil  water  and  thus  drive  out 
the  air  dissolved  in  it,  fishes  and  other  aquatic 
animals  soon  die.     But  plants  are  no  less  dependent 


METABOLISM  63 

upon  a  supply  of  oxygen.  If  a  transparent  cell  of 
plant  tissue,  in  which  the  protoplasm  is  in  active 
streaming  movement,  be  so  mounted  that  the  sur- 
rounding air  can  be  replaced  by  pure  hydrogen,  the 
streaming  will  cease  entirely  until  oxygen  is  again 
supplied  (the  hydrogen  is  itself  inert  toward  pro- 
toplasm). Plants  likewise  cease  to  grow  in  the 
absence  of  oxygen.  The  presence  of  this  element, 
thus  appears  to  be  an  absolute  necessity  for  life  as  it 
exists  on  the  earth  to-day. 

But  an  important  exception  to  this  statement  must 
be  noted.  A  number  of  the  lower  plant  forms  have 
been  found  to  thrive  in  an  absence  of  oxygen  and  in- 
deed to  refuse  to  grow  in  its  presence.  Such  forms, 
which  include  numbers  of  the  bacteria,  are  called 
anaerobic.  Some  bacteria  are  able  to  adapt  them- 
selves to  either  the  presence  or  the  absence  of  oxygen ; 
others  can  thrive  only  in  the  absence  of  it.  The 
former  are  termed  facultative  anaerobes,  the  latter 
obligate  anaerobes.  Among  the  latter  are  numbered 
some  of  the  most  dreaded  disease-producing  or 
pathogenic  organisms.  And  in  this  connection 
their  anaerobic  habit  is  of  much  significance.  The 
germ  of  tetanus  (lockjaw),  for  example,  is  very 
widely  distributed,  and  the  only  reason  that  the 
disease  is  not  much  more  frequent  than  it  is,  is  that 
the  spores  of  the  active  agent  cannot  develop  in  the 
presence  of  oxygen,  and  hence  only  those  wounds 
that  are  deep  and  that  close  over,  thus  excluding  the 
air,  are  likely  to  afford  sites  for  the  development 
of  the  poison-producing  bacteria. 


64  GENERAL  BIOLOGY 

Combustion  and  Respiration.  —  Since  oxygen  is 
so  significant  in  organic  life,  it  is  important  to 
find  out  what  role  it  plays  in  metabolism.  It  was 
among  the  earlier  discoveries  of  modern  chemis- 
try that  when  anything  is  burned,  the  combustion 
involves  a  using-up  of  oxygen  (oxidation)  and 
will  not  take  place  in  the  absence  of  oxygen. 
When  wood  is  burned,  the  carbon  in  it  unites 
with  the  oxygen  to  form  CO2,  and  the  hydrogen 
to  form  H2O  or  water.  It  is  easy  to  observe 
that  in  plants  as  well  as  in  oxygen-breathing 
animals  not  only  is  O  taken  in,  but  CO2  is 
expelled.  This  interchange  is  known  as  respira- 
tion, and  it  was  an  obvious  step  to  compare 
it  with  ordinary  combustion,  particularly  as  the 
production  of  the  bodily  heat  of  the  higher  ani- 
mals is  unquestionably  dependent  upon  oxida- 
tions. From  this  standpoint,  the  foods  which 
the  organism  takes  into  itself  were  supposed  to 
be  oxidized  with  an  evolution  of  energy,  in  the 
same  way  that  the  fuel  burnt  under  an  engine 
boiler  generates  steam  to  drive  the  wheels  of  the 
engine. 

In  the  burning  of  fuel  the  oxygen  supplied  by  the 
draft  combines  directly  with  the  fuel,  but  it  is  not 
difficult  to  show  that  the  C02-production  of  an 
animal  or  plant  bears  no  direct  relation  at  all  to  the 
intake  of  oxygen.  A  frog  in  an  atmosphere  of  hy- 
drogen will  continue  to  evolve  CO2  without  any 
possible  supply  of  gaseous  oxygen.  The  CO2  in 
such    a    case    must    have   been    evolved    as    a    by- 


METABOLISM  65 

product  of  changes  taking  place  in  the  tissue  sub- 
stance itself.^ 

These  reactions,  taking  place  constantly  in  the 
tissues,  are  obviously  of  a  very  different  sort  from 
the  exchange  of  gases  (breathing)  to  be  observed  in 
higher  plants  and  animals.  They  constitute  the 
true  respiratory  process.     But  the  term   respiration 

1  Chemists  have  discovered,  indeed,  that  dry  oxygen,  at  low  tempera- 
tures, is  the  greatest  retarding  agent  in  combustion.  We  are  familiar 
with  the  fact  that  iron  rusts  much  more  quickly  if  wet  than  if  dry,  and  if 
kept  perfectly  dry,  will  not  rust  at  all.  The  rusting  is  an  oxidation  or 
slow  combustion  resulting  from  the  combination  of  the  metal  with  oxygen 
to  form  iron  oxide.  In  this  case  the  reactions  are  perhaps  as  follows 
(Matthews) : 

I.     Fe  +  2  H2O  =  Fe(OH)2  +  2H 
II.     4  H  +  O2       =2  H2O,  or 
2  H  +  O2       =  H2O2 
H2O2  +  Fe     =  FeO  +  H2O. 

In  the  first  equation  the  iron  combines  with  the  water  to  form  ferrous 
hydrato  and  hydrogen.  The  latter  would  immediately  reduce  the  former 
to  metallic  iron  again  if  there  were  not  oxygen  present  with  which  it 
can  combine  to  form  hydrogen  peroxide,  which,  giving  up  its  extra  atom 
of  O,  forms  ferric  oxide,  or  iron  rust,  and  water.  The  oxygen  acts  thus 
not  as  a  direct  combining  agent  with  the  iron,  but  rather  as  a  sort  of 
depolarizer  to  take  off  the  nascent  hydrogen,  and  the  oxidation  of  the 
iron  is  effected  by  the  hydrogen  peroxide. 

It  is  supposed  that  in  animal  and  plant  tissues  much  the  same  sort 
of  thing  takes  place,  but  with  infinitely  more  complicated  reactions. 
The  essential  point,  however,  is  that  the  oxygen  does  not  combine 
directly  with  the  carbon  element  of  the  protoplasm  to  form  CO2,  but, 
where  water  enters  into  reactions  with  the  substances  composing  the 
tissues,  it  acts  as  a  sort  of  depolarizer  to  combine  with  the  hydrogen 
liberated.  The  CO2  probably  arises  independently  as  a  by-product 
in  the  shifting  and  rearrangement  of  various  components  of  the  sub- 
stances making  up  protoplasm.  "It  is  a  sort  of  receipt  for  a  given 
amount  of  energy  released  by  chemical  decomposition." 


66  GENERAL  BIOLOGY 

is  so  firmly  associated  with  the  external  phenomena 
just  mentioned  that  its  use  in  connection  with  the 
oxidations  and  reductions  taking  place  in  tissues  is 
apt  to  be  misleading.  For  this  reason  the  term 
Energesis  has  been  proposed  for  tissue  respiration. 
Since  the  function  of  the  process  is  to  release  the 
necessary  energy  required  in  the  life  of  the  organism, 
this  word  is  very  appropriate  and  deserves  wider  use. 

Poisons.  —  So  long  as  katabolism  is  compensated 
by  an  approximately  equal  anabolism,  the  life  process 
proceeds  normally.  If  the  destruction  of  proto- 
plasm, however,  occurs  too  rapidly  for  the  con- 
structive changes  to  keep  up  with  it,  then  abnormal 
conditions  arise  which  soon  result  in  the  death  of 
the  tissue  or  of  the  organism.  Thus,  although 
oxidations  are  absolutely  essential  for  life,  excessive 
oxidation  soon  destroys  the  protoplasm,  and  certain 
oxidizing  substances,  such  as  potassium  perman- 
ganate, are  poisons  on  this  account.  Other  sub- 
stances, such  as  the  salts  of  the  heavy  metals, 
mercury,  silver,  etc.,  destroy  the  life  of  the  proto- 
plasm by  entering  into  permanent  combination  with 
substances  composing  it.  Other  more  complex 
substances  act  as  poisons  by  substituting  themselves 
for  essential  parts  of  the  protoplasm.  Thus  many  of 
the  proteins  elaborated  by  plants.,  such  as  strychnine, 
morphine,  caffeine,  etc.,  are  deadly  poisons  to  the 
tissues  of  higher  animals.  The  actions  of  these 
substances  are,  however,  very  diverse.  Indeed, 
the  animal  organism  constantly  forms  such  protein- 


METABOLISM  67 

like  or  nitrogenous  compounds  as  by-products  of 
normal  metabolism,  and  these,  unless  removed  by 
excretion,  poison  and  eventually  kill  the  organism 
that  produces  them. 

Another  group  of  substances,  called  anaesthetics, 
of  which  chloroform  and  ether  are  the  most  familiar, 
depress  the  activities  of  protoplasm  and,  if  not 
counteracted,  kill  it.  The  action  of  such  substances 
is  still  a  matter  of  debate,  but  since  all  of  them  are 
fat  solvents,  it  has  been  supposed  that  their  poisonous 
action  may  be  exerted  on  the  fatty  components  of  pro- 
toplasm. The  poisonous  action  of  these  substances 
is  of  a  different  character  from  the  depressant  action 
mentioned.  If  the  action  of  the  anaesthetic  is  not 
too  violent  or  too  prolonged,  the  protoplasm  will 
later  recover  its  activities  and  resume  its  functions 
as  before.  It  has  been  shown  that,  whereas  the 
action  of  poisons  (including  such  substances  as 
ether  and  chloroform  in  poisonous  doses)  greatly 
increases  the  permeability  of  the  cell  membrane, 
the  merely  anaesthetic  effect  is  accompanied  by  a 
temporary  decrease  of  permeability. 

Antiseptics.  —  Nearly  all  the  disease  and  physical 
suffering  that  man  is  subject  to  is  the  result  of  the 
activity  of  microorganisms  that  find  lodgment 
within  the  body,  and,  rapidly  multiplying,  produce 
poisons  that  affect  the  whole  system.  We  combat 
these  by  the  use  of  such  poisons  as  have  been  men- 
tioned above,  —  chemicals  that  either  inhibit  the 
growth  of  the  organisms  or  destroy  them.     Especially 


68  GENERAL  BIOLOGY 

useful  are  bichloride  of  mercury,  alcohol,  and  phenol 
(carbolic  acid). 

The  Cycle  of  the  Elements  in  Organic  Nature.  — 
The  ultimate  source  of  the  carbohydrates  and  fats 
in  plants  and,  hence,  secondarily  in  animals,  is,  as 
we  have  seen,  the  carbon  existing  as  CO2  in  the 
atmosphere.  There  is  reason  to  suppose  that  in 
geologic  time  past,  owing  probably  to  great  volcanic 
activity  then,  the  amount  of  CO2  in  the  atmosphere 
was  much  greater  than  it  is  at  present.^  At  any  rate 
the  proportion  of  CO2  in  the  air  nowadays  is  sur- 
prisingly small,  —  not  more  than  .05  per  cent,  of 
which  the  carbon  itself,  the  part  utilized  by  the 
plant,  constitutes  but  a  little  more  than  one  fifth. 
The  botanists  have  calculated  that  the  cellulose  in 
a  single  dried  tree  trunk  weighing  11,000  lbs.,  repre- 
sents a  carbon  moiety  of  5500  lbs.,  and  that  to  secure 
this  amount  of  carbon  such  a  tree  must  have  drained 
more  than  16,125,000  cubic  yards  of  air  of  its  CO2. 
Meteorologists  calculate  that,  although  the  atmos- 
pheric envelope  of  the  earth  may  extend  (in  a  very 
tenuous  state)  several  hundred  miles  above  the  sur- 
face, yet  seven  eighths  of  it  by  weight  lies  under  a 
height  of  10.2  miles  from  the  ground.  Allowing 
for  the  diffusion  of  its  constituents,  we  may  estimate, 
for  the  sake  of  the  argument,  that  the  surface  vege- 
tation can  draw  tribute  of  CO2  from  a  height  of  ten 

^  Geologists  have  even  ascribed  the  initiation  of  the  glacial  epochs 
to  the  decrease  of  CO2  in  the  atmosphere,  consequent  in  part  upon  the 
great  development  of  plant  life  in  preceding  epochs.  See  Chamberlain 
and  Salisbury,  Geology,  III,  p.  424. 


METABOLISM  69 

miles.  The  surface  of  the  state  of  Oregon  is  given 
as  94,560  square  miles,  that  is  to  say,  945,600  cubic 
miles  of  air  cover  that  densely  wooded  state.  On 
the  basis  of  the  calculation  made  in  the  last  paragraph 
this  would  allow  but  338.1  trees  of  the  sort  mentioned 
to  a  cubic  mile,  or  3381  to  a  square  mile  of  surface, 
or  one  tree  to  every  918  square  yards,  —  a  number 
which  is  probably  exceeded  many  times  in  any  one 
generation. 

Of  course,  both  plants  and  animals  "  breathe  out  " 
quantities  of  CO2  in  respiration  as  described  in  the 
previous  section,  yet  such  amounts  must  be  far 
inadequate  to  make  good  the  loss  to  the  atmosphere 
through  plant  growth. 

Since  we  have  seen  that  the  carbon  compounds 
in  the  soil  cannot  be  utilized  by  the  plants  to  build 
up  carbohydrates,  it  is  evident  that  the  air  must 
be  constantly  supplied  with  quantities  of  CO2  from 
some  source.  Before  we  follow  this  farther ^  let 
us  consider  for  a  moment  the  nitrogen  balance 
sheet. 

The  plant  draws  from  the  soil  all  the  requisites 
for  its  protein,  and,  hence,  for  the  bulk  of  its  proto- 
plasm. The  rapidity  with  which  a  crop,  say 
of  wheat,  can  exhaust  the  available  and  necessary 
mineral  food  in  the  soil  has  been  frequently  and 
strikingly  demonstrated  in  America,  where  the 
originally  rich  virgin  soils  have  been  repeatedly 
"  robbed  "  and  then  abandoned  for  other  un worked 
fields.  Only  recently,  when  the  supply  of  free 
land  is  reaching  its  end,  has  pressure  been  brought 


70 


GENERAL  BIOLOGY 


to  bear  on  the  agriculturist  to  replace  by  fertilizers 
what  his  crops  have  drawn  out  of  the  soil. 

It   is   obvious   that   the   materials   thus   removed 
depend  largely  upon  the  kind  of  plant  that  is  grow- 
ing, each  kind  drawing  out  certain 
things  for  its  own  particular  needs. 
Yet  there  are  some  mineral  com- 
pounds that  are  demanded  by  all 
plants,    the   absence  of  which  in- 
terferes with  or   prevents   normal 
growth.     These,  as  has  been  noted 
already,  are  the  metal  salts 
of   phosphoric,    nitric,  and 
sulphuric  acids,  usually  po- 
tassium     phos- 
phate, potassium 
nitrate,    and    cal- 
cium   and     mag- 
nesium sulphates. 
It   is    possible  to 
make  a  solution^ 
of    a    mixture    of 
these    salts    in 
which  plants  will 
thrive.      The  ab- 
sence of  any  of  the  above-mentioned  elements  in 
such  a  solution  induces  marked  disturbances  in  the 
normal  growth  of  the  plant. 

^  The  following  solution,  devised  by  Schimper,  has  been  much  used  in 
experimental  work :  calcium  nitrate,  6  gm. ;  potassium  nitrate,  15  gm. ; 
magnesium  nitrate,  15  gm. ;  potassium  phosphate,  15  gm. ;  sodium  nitrate, 
1.5  gm. ;  distilled  water,  600  cc,  to  which  is  added  a  trace  of  ferric  chloride. 


Fig.  23.  —  Cultures  of  hemp  grown  in 
neutral  solid  substratum.  A  complete  nu- 
trient solution  has  been  added  to  /,  and  the 
plants  have  attained  a  height  of  1.5  meters  ; 
a  solution  lacking  potassium  nitrate  has  been 
added  to  the  substratum  in  //,  and  only  the 
sterile  substratum  placed  in  the  pot  in  III. 
—  (From  MacDougle,  after  Ville.) 


METABOLISM  71 

The  amount  of  nitrogen  taken  from  the  soil,  annu- 
ally, by  an  average  crop  in  Alsace  was  estimated 
at  approximately  46  lbs.  to  the  acre.  Less  than 
half  this  amount  is  returned  directly  to  the  soil 
(mostly  as  volatile  ammonia).  Moreover,  every 
inch  of  rain  falling  on  the  land  and  draining  through 
it  causes  an  additional  loss  of  something  like  2|  lbs. 
of  nitrogen  to  the  acre.  There  must  be  an  excess  of 
nitrogen,  therefore,  in  the  soil  over  and  above 
what  the  plant  life  demands.  Here  again,  as  was 
the  case  with  the  carbon,  the  demand  would  seem  to 
greatly  exceed  the  supply. 

Since  man  first  began  to  develop  the  art  of  agricul- 
ture, he  has  practiced  various  methods  of  replenishing 
the  soil  from  which  his  crops  have  taken  their  foods. 
Especially  has  he  used  various  sorts  of  manures, 
which  contain  nitrates  and  phosphates.  Modern 
man  has  added  to  these  fertilizers  various  mineral 
substances  which  he  quarries  from  the  earth,  such 
as  phosphate  of  lime  and  "  potash  "  (potassium 
phosphates  and  sulphates).  The  so-called  "  basic 
slag,"  a  residue  from  metal  smelting,  which  contains 
12-20  per  cent  of  phosphoric  acid,  is  nowadays 
finely  ground  and  largely  used  to  replenish  the  supply 
of  phosphates.  The  weathering  of  the  rocks  also 
slowly  adds  to  the  soil  the  soluble  components 
needed  by  organic  life,  and  of  course  this  was  their 
original  source.  But  whatever  man  can  return  to 
the  soil  is  obviously  insignificant  compared  with 
what  Nature's  crops  remove.  The  huge  stores 
of   nitrogen   and   carbon   constantly   built   up   into 


72 


GENERAL  BIOLOGY 


plant    and    animal    tissue    must    be,    somehow,    as 
constantly  restored. 

Destruction  of  Organisms.  —  Each  year  sees  a 
prodigious  crop  of  annual  plants  —  "  weeds  "  — 
in  every  vacant  lot.  Of  course  a  large  part  of  their 
bulk  is  water,  yet  even  if  dried  out,  the  accumulations 
of  a  few  years,   if  preserved,   would  so  cover  the 


PLANT  TISSUES 


ATMOSPHERE 


ANIMAL  TISSUES 


Fig.  24. 


DECAY- 

Diagram  of  the  carbon  cycle  in  organic  and  inorganic  nature. 


ground  as  to  make  it  impossible  for  other  plants  to 
struggle  up  from  the  soil  to  the  light.  The  repro- 
ductive capacity  of  nearly  all  animals  is  astonishing, 
and  the  struggle  for  existence  entails  the  destruction 
of  hosts  of  individuals  every  year.  Yet  we  do  not  find 
the  surface  of  the  earth  cluttered  with  dead  animals. 
Indeed  we  rarely  see  one  at  all,  or,  if  we  do,  it  is  only 
the  whitening  bones  which  the  rains  are  slowly  wash- 
ing away.  What,  then,  becomes  of  them  ?  The  an- 
swer is  at  every  hand.  Wood  is  more  resistant  than 
animal    tissue,    and    we    can    everywhere    observe 


METABOLISM  73 

the  phenomenon  of  its  slowly  rotting  away.  With 
animal  remains  the  process  is  much  more  rapid. 
In  the  end,  however,  the  myriads  of  animdl  and 
plant  individuals,  after  their  brief  existence  on  the 
earth,  dissolve  back  again  into  the  elements  from 
which  they  were  built  up.  "  Dust  to  dust  "  is  a 
very  real  and  constantly  recurring  cycle.  The  com- 
plex substances  composing  living  tissue  by  successive 
cleavages  become  resolved  into  H2O,  CO2,  NH3,  and 
similar  compounds.  Were  this  not  so,  the  material 
to  build  the  organic  world  would  soon  be  exhausted, 
and  the  earth  would  be  so  covered  with  the  remains 
of  animals  and  plants,  accumulating  during  long 
periods  of  time,  that  life  would  be  impossible.  In 
very  hot,  dry  climates,  as  in  deserts,  animal  remains 
actually  do  dry  up  and  "  mummify,"  without  de- 
caying. In  the  presence  of  moisture,  however, 
dissolution  begins  as  soon  as  life  is  extinct. 

Putrefactive  Organisms.  —  This  process  of  dis- 
solution, which  is  merely  the  cleavage  of  the  proto- 
plasmic compounds  into  their  simpler  components, 
doubtless  would  go  on  automatically,  at  least  to  a 
certain  point,  but  the  process  is  hastened  and  carried 
to  a  final  conclusion  by  the  assistance  of  various 
kinds  of  bacteria  and  molds,  known  as  the  putre- 
factive organisms.  In  the  light  of  what  has  just 
been  said  these  obscure  and  unpleasant  atoms  of 
life  are  one  of  the  most  important  agents  in  the  econ- 
omy oi  nature.  They  are  the  wreckers,  tearing 
down  that  Nature  may  build  herself  anew.     Through 


74  GENERAL  BIOLOGY 

their  action,  carbohydrates  and  fats  are  reduced  to 
CO2  and  H2O,  and  proteins  are  dissolved,  by  a  long 
chain  of  reactions,  among  other  things  into  NH3  and 
CO2.  The  urea  excreted  by  animals  goes  the  same 
way.  By  this  means  the  rotation  of  the  elements 
through  organic  and  inorganic  nature  is  hastened  and 
facilitated .  The  ammonia  in  the  soil  is  taken  in  hand 
by  another  group  of  bacteria,  the  nitrite  bacteria, 


Fig.  25.  —  Putrefactive  bacteria  of  various  sorts  :  A,  Bacillus  urea, 
the  agent  that  ferments  urea  into  ammonium  carbonate  and  water  and 
eventually  into  carbon  dioxide  and  ammonia  ;  B,  Bacillus  subtilis,  a  putre- 
factive organism  commonly  found  in  hay  infusions  ;  C,  the  same  in  the 
inactive  zoogloea  condition  ;  D,  Spirilla,  sp.,  from  hay  infusion  ;  E,  Bac- 
terium "  termo,"  from  fermenting  infusion  of  peas. 

which  oxidize  it  to  nitrous  acid  (HNO2).  This 
combines  with  potassium  or  ammonia  in  the  soil  to 
form  potassium  or  ammonium  nitrite.  Another 
group  then  further  oxidizes  the  nitrite  into  nitrate,^ 
and  makes  it  available  for  plant  food.  There  is 
a  similar  cycle  for  the  sulphur  and  phosphorus, 
but  these  elements,  although  absolutely  essential 
for  organic  life,  are  but  a  small  fraction  of  the  bulk 

1  I.    2  NH3  +  3  O2  =  2  HNO2  +  2  HoO. 
II.  2  HNO2  +  O2  =  2  HNO3 


METABOLISM  75 

of  the  carbon,  hydrogen,  •  and  nitrogen  in  living 
things,  and  in  consequence  there  is  little  danger  of 
the  supply  ever  becoming  exhausted.  The  purpose 
of  phosphate  fertilization  is  to  supply  the  demands 
of  the  nitrogen-fixing  bacteria  {Azotohacter) y  rather 
than  the  green  plants  themselves,  and  thus  to  aid 
the  process  of  nitrogen  fixation  (see  below). 


Denitrification.  —  The  circle,  nevertheless,  is  not 
so  ideal  as  it  might  seem,  for  there  exists  another 
group  of  bacteria  in  the  soil  whose  special  activity 
it  is  to  reduce  nitrates  again  to  gaseous  nitrogen, 
which  escapes  to  the  atmosphere  and  is  added  to 
the  inert  quantity  that  plants  are  unable  to  "  fix:  " 
or  utilize.  This  means  a  constant  loss  of  available 
nitrogen,  a  constant  deficit,  so  to  speak,  in  the  annual 
balance  sheet,  and  when  the  facts  first  became 
known,  considerable  doubt  was  expressed  as  to  the 
future  habitability  of  the  earth  when  the  available 
nitrogen  should  have  been  reduced  too  far.  For- 
tunately for  our  peace  of  mind,  there  have  been  dis- 
covered yet  other  kinds  of  bacteria  and  molds  that 
are  capable  of  fixing,  that  is,  combining  with  the 
nitrogen  of  the  soil.  These  seem  to  be  everywhere 
present  in  soils  and  make  up  for  the  loss  due  to  the 
denitrifying  bacteria. 

It  has  been  known  for  many  centuries  that  it  im- 
proves the  land  to  grow  crops  of  peas,  beans,  or  their 
relatives,  and  plow  them  under  as  "  green  manure." 
The  ancient  Romans  and  the  Chinese  and  Japanese 
carried    out   such   practices    without    knowing    any 


76 


GENERAL  BIOLOGY 


reason  for  the  improvement.  It  has  been  known  for 
a  long  time,  too,  that  the  rootlets  of  leguminous 
plants  (i.e.  peas,  beans,  alfalfa,  etc.)  are  beset 
with  little  nodules  or  tubercles,  which  were  supposed 


.4^\-. 

V  "^ 

It-    '' 

^m 

^J 

y     Y 

"W 

^ 

^ 

Fig.  26.  —  Tubercles  on  roots  of  clover. —  (Osterhout.) 

to  be  pathological  growths  of  the  nature  of  galls. 
Comparatively  recently  it  has  been  demonstrated 
that  these  tubercles  harbor  minute  bacteria  endowed 
with  the  property  of  fixing  atmospheric  nitrogen, 
and  that  this  is  the  chief  reason  for  the  value  of  such 
plants  as  fertilizers.     We  discover,  therefore,  that 


METABOLISM  77 

life  not  only  goes  on  upon  the  earth  but  in  the  earth 
as  well,  and  that  the  soil,  far  from  being  the  lifeless 
mass  of  rocks  and  dirt  we  are  accustomed  to  con- 
sider it,  is  pulsing  with  life  in  every  granule,  harbor- 
ing a  multitude  of  different  organisms  that  live  in 
darkness  and  for  the  most  part  without  oxygen, 
but  whose  activities  are  so  vital  for  the  more  familiar 
life  above  the  ground  that  one  could  not  exist  without 
the  other. 

Nature  of  the  Energy  Transformed.  —  Move- 
ment. —  The  latent  energy  of  chemical  affinity 
may  be  transformed  into  all  the  other  forms  of 
energy  known.  The  movements  of  an  animal  are 
primarily  brought  about  by  the  contractility  of  its 
protoplasm,  which  is  almost  always  differentiated 
(except  in  Protozoa)  as  muscular  tissue. 

Intracellular  circulation  has  been  previously  men- 
tioned. This  is  a  form  of  motion  probably  univer- 
sal in  animals  and  plants.  The  simplest  form  of 
motion,  after  this,  is  that  described  in  the  previous 
chapter  in  connection  with  the  movements  of 
leucocytes,  and  known  as  amoeboid,  because  it  is 
the  characteristic  and  only  form  of  locomotion 
possessed  by  Amoeba.  A  similar  mode  of  unlocalized 
movement  is  also  found  in  many  pigment  cells  and 
in  the  primitive  connective  tissue  cells  of  the  develop- 
ing embryo. 

The  majority  of  the  Protozoa  move  by  means  of 
special '  organs  of  locomotion,  differentiated  either 
as  cilia,  covering  all  or  parts  of  the  cell-body,  or  as 


78  GENERAL   BIOLOGY 

flagella,  inserted  at  the  end  or  side  of  the  cell.  It  is 
supposed  that  both  cilia  and  flagella  are  hollow 
extensions  of  the  cuticle,  which  by  a  sudden  contrac- 
tion on  one  side  produce  a  resultant  movement 
like  that  of  a  fishing  pole  when  a  fly  is  cast.  Fixed 
ciliated  cells  are  also  found  as  components  of  many 
tissues  in  Metazoa,  as  in  the  nasal  passages  of  verte- 
brates.    By  the   force  of  the   beating  cilia  currents 


Fig.  27.  —  The  electric  ray  (Torpedo)  ;  the  skin  partially  cut  away  so 
that  the  electric  organ,  a,  is  visible  ;  it  consists  of  numerous  polygonal 
columns  of  modified  muscular  tissue.  —  (From  Verworn,  after  Ranvier.) 

of  liquid   are  urged  along  over  the  surface  of  the 
epithelium. 

The  most  highly  developed  tissue  of  locomotion 
is,  of  course,  striated  muscle.  When  a  muscle  con- 
tracts, it  shortens  and  thickens  without  changing 
its  bulk.  Movement  is  communicated  through  its 
fixed  tendons  to  whatever  bones  or  other  structures 
it  may  be  attached.  When  a  muscle  has  made  a 
number  of  contractions,  it  shows  a  marked  increase 
in  acidity.     This  is  due  to  the  appearance  of  sar- 


METABOLISM 


79 


colactic  acid,  which  can  be  shown  to  be  a  product 
of  the  breaking  down  of  substances  in  the  muscle 
fiber. 

Heat.  —  During  contraction  the  muscle  also  de- 
velops heat,  and  it  has  been  estimated  that  the 
amount  of  energy  liberated  as  heat  is  five  times  that 
utilized  as  mechanical  energy,  i.e.  "  work  "  in  the 
ordinary  sense.  This  is  shown  by  the  "  warming 
up "  that  physical  exercise 
brings.  The  contracting  mus- 
cle liberates  still  a  third  form 
of  energy,  namely,  electricity, 
which  may  be  measured  by  a 
capillary  electrometer. 

Electricity.  —  Both  heat  and 
electricity  as  accompaniments 
of  muscular  activity  are,  in  a 
sense,  waste  products  and  repre- 
sent a  certain  necessary  loss. 
On  the  other  hand,  heat  is  a 
transformation  of  energy  very 
necessary  to  the  organism,  not  only  in  the  higher 
animals,  where  a  definite  bodily  temperature  must 
be  kept  up,  but  in  all  animals  and  plants  as 
well,  since  it  is  only  in  the  presence  of  a  cer- 
tain amount  of  heat  that  the  necessary  oxidations 
and  reductions  involved  in  metabolism  can  take 
place.  We  are  all  familiar  with  the  rise  of  tempera- 
ture produced  by  fermenting  yeast.  In  both  animals 
and  plants  the  evolution  of  heat  is  usually  brought 
about  by  the  cleavage  of  a  carbohydrate. 


Fig.  28.  —  Nocttluca, 
a  phosphorescent  marine 
Protozoan;  magnified  30 
diameters.  —  (From 
Weysse  after  Cienkowski.) 


80  GENERAL  BIOLOGY 

Static  electricity  is  probably  always  developed  by 
both  animals  and  plants,  as  a  universal  concomitant 
of  vital  activity,  but  in  some  forms,  such  as  the 
electric  eel  and  electric  ray  (Torpedo),  an  arrange- 
ment of  muscle  fibers,  in  the  structure  of  a  galvanic 
pile,  permits  of  the  accumulation  of  a  charge,  so 
that  such  an  animal  can  give  a  severe  shock. 


Fig.  29.  —  Phosphorescence  in  Noctiluca.  A  portion  of  the  body  is 
represented,  with  numerous  scintillating  dots.  —  (From  Calkins,  after 
Quatrefages.) 

Light.  —  The  energy  transformations  of  metabo- 
lism also  occasionally  take  the  form  of  light.  This 
is  often  called  phosphorescence,  owing  to  the  fact 
that  the  light  usually  resembles  the  glow  of  phos- 
phorus. As  a  matter  of  fact  it  has  nothing  to  do 
with  phosphorus,  which  in  its  free  and  luminous 
state  is  an  active  poison  to  all  living  protoplasm. 
Phosphorescence  is  a  special  characteristic  of  many 
minute  organisms  of  the  sea  and  of  the  bacteria 
that  develop  in  decaying  wood  and  fish.  Some  of 
the  more  complex  animals  are  provided  with  special 
light-giving  organs  that  flash  in  the  dark  like  torches. 
Certain  insects  show  a  remarkable  development  in 


METABOLISM  81 

this  direction.  The  most  familiar  example  is  that 
of  the  firefly,  which  has  light-giving  organs  at  the 
base  of  the  abdomen.  In  the  depths  of  the  sea 
many  of  the  fishes  that  inhabit  those  abysses  have 
similar  light-producing  spots  distributed  over  the 
body  in  characteristic  ways.  One  of  the  species 
even  develops  such  an  organ  at  the  end  of  a  fila- 
ment like  a  pendent  incandescent  lamp. 


Fig.  30.  —  Lantern  fish  (Linophryne  lucifer).  The  phosphorescent 
bulb  doubtless  functions  as  a  lure  to  entice  other  fishes  within  reach  of 
the  jaws.  The  body  is  distended  with  a  large  fish  that  has  been 
swallowed.  —  (After  Collet.) 

It  has  been  found  that  the  efficiency  of  the  firefly's 
light  is  practically  100  per  cent,  that  is,  none  of  the 
energy  is  lost  as  heat ;  whereas  in  an  ordinary  in- 
candescent lamp  but  3i  per  cent  is  utilized  as  light, 
and  in  an  arc  but  15  per  cent,  the  rest  of  the  energy 
being  wasted  as  heat.  In  the  insects  mentioned 
above,  the  glow  is  produced  by  the  oxidation  of 
some  specially  secreted  substance,  probably  fatty  in 
nature,  and  the  flashes  of  the  firefly  correspond 
with  the  intervals  of  taking  in  air  through  the  res- 
piratory tubes. 


GENERAL  BIOLOGY 


In  addition  to  the  foregoing  forms  of  energy 
protoplasm  utilizes  its  latent  energy  to  produce 
the  characteristic  products  that  have  already  been 

described.  These 
include,  besides 
the  food  reserves, 
such  as  fat,  starch 
granules,  egg- 
yolk,  etc.,  minute 
quantities  of 
other  substances 
(zymogens  and 
hormones)  which 
enable  the  cell 
to  accomplish  its 
multitudinous  re- 

v^im  ^m                     .^^^^           actions  with  the 
vt»-   s                t    J^^^K          minimum  ex- 
penditure  of  en- 
ergy. 

Enzymes  and 
Enzymotic  Reac- 
tions.— If  starch 
be  taken  into  the 
mouth  or  placed 
in  a  test  tube  with 
saliva,  it  is  acted  upon  by  the  saliva  and  its  mole- 
cules are  split  into  their  sugar  elements,  in  which 
form  they  may  be  absorbed  through  the  lining  wall 
of  the  digestive  tube.     The  chemist  can  also  boil 


Fig.  31.  —  The  firefly;  the  one  on  the 
ground  shows  the  phosphorescent  organs  (the 
three  white  segments  of  the  abdomen). — 
(From  Linville  and  Kelly.) 


METABOLISM  83 

the  starch  in  any  strong  acid  and  produce  the  same 
change.  In  this  case,  however,  a  strong  reagent 
and  a  high  degree  of  heat  are  necessary.  The  mildly 
alkaline  saliva  at  ordinary  temperature  thus  seems 
to  be  able  to  accomplish  the  same  result  with  a 
minute  fraction  of  the  expenditure  necessary  in 
the  second  experiment. 

The  prompt  and  effective  action  of  the  saliva  is 
due  to  the  presence  therein  of  a  minute  quantity  of 
a  substance  called  ptyalin,  one  of  a  large  group  of 
somewhat  similar  substances  which  we  call  enzymes. 
A  similar  enzyme,  called  pepsin,  splits  up  proteins 
in  the  stomach ;  another,  lipase,  splits  fats,  in  various 
parts  of  the  body;  another  clots  blood,  and  so  on. 
None  of  these  enzymes  has  been  isolated  in  a  pure 
state,  ami  we  know  little  of  their  composition,  al- 
though it  has  been  surmised  that  they  are  allied  to 
the  proteins.  They  are  certainly  not  alive  in  the 
usual  sense  of  the  word,  for  they  may  be  precipitated 
without  injury  with  absolute  alcohol  and  other 
reagents.  They  are,  however,  destroyed  by  boiling 
and  do  not  "  work  "  well  below  a  certain  minimum 
temperature.  Most  of  them  need  a  special  environ- 
ment {e.g.  hydrochloric  acid  for  pepsin)  to  work 
best,  and  each  of  them  is  specific  in  its  action; 
that  is,  affects  only  one  kind  of  substance.  The 
most  remarkable  thing  about  them,  however,  is 
that  although  a  minute  quantity  will  produce  a 
relatively  enormous  effect,  the  enzyme  does  not  ap- 
pear to  be  used  up  at  all,  and  the  process  may  go  on 
indefinitely,  provided  the  products  of  its  action  are 


84  GENERAL  BIOLOGY 

removed  as  soon  as  formed.  Some  of  these  enzymes 
are  reversible,  splitting  and  synthesizing  the  same 
substance,  as  conditions  differ.  In  these  processes, 
enzymes  seem  to  follow  the  same  "  law  of  mass 
action  "  as  well  as  the  equations  of  reaction- velocity 
in  relation  to  temperature  that  have  been  worked 
out  in  inorganic  chemistry. 

Within  the  cell,  through  the  aid  of  enzymes  there 
present,  analysis  and  synthesis  may  take  place 
side  by  side,  and  by  a  series  of  such  changes,  oxida- 
tion following  reduction  in  continuous  sequence,  the 
most  complex  molecules  may  be  built  up,  the  energy 
of  one  disruptive  process  being  utilized  to  combine 
the  components  into  a  molecule  of  a  higher  order. 
Oxidizing  enzymes  or  "  oxidases  "  seem  to  be 
present  in  all  protoplasm,  and  owing  to  their  presence 
the  necessary  oxidations  take  place  in  the  organism 
very  rapidly  at  a  comparatively  low  temperature. 
The  seat  of  these  oxidations,  and  hence  of  the  oxi- 
dative enzymes,  appears  to  be  in  the  nucleus. 

A  distinction  was  formerly  made  between  the 
so-called  "  unorganized  ferments,"  such  as  pepsin, 
and  the  "  organized  ferments,"  such  as  the  active 
principle  of  yeast,  which  was  supposed  to  require 
the  presence  of  a  living  cell  in  order  to  work.  It 
has  been  found  possible,  however,  to  crush  all  life 
out  of  the  yeast  cells  and,  by  filtering  the  extract,  to 
get  a  solution  which  is  as  active  a  fermentative 
agent  as  living  yeast.  The  distinction  between  the 
two  sorts  of  enzymes  then  falls  to  the  ground. 
The  only  difference  seems  to  be  that  some  enzymes 


METABOLISM  85 

work  within  the  cell  and  others  without.  The  idea 
that  the  action  of  enzymes  is  physical  and  mechani- 
cal is  strengthened  by  the  fact  that  finely  divided 
platinum  (platinum  black)  will  produce  catalytic 
effects  similar  to  those  produced  by  oxidizing  en- 
zymes. 

The  most  wonderful  feature  of  all,  perhaps,  is 
the  fact  that  the  protoplasm  makes  its  own  enzymes, 
since  they  are,  of  course,  the  secreted  products  of 
the  activity  of  the  living  substance,  developed  in 
just  the  places  and  apparently  at  just  the  time  when 
needed. 

Since  the  action  of  enzymes  is  mechanical,  the 
question  has  often  arisen,  —  How  is  it  controlled  ? 
Why,  for  example,  does  not  the  stomach  digest 
itself.'^  We  hardly  know  enough  about  enzymes  to 
answer  such  a  question  in  detail,  but  we  have  learned 
that  many  enzymes  when  produced  are  incapable  of 
performing  their  offices  until  supplemented  by 
another  element,  usually  produced  in  a  different 
region.  The  pancreatic  secretion  has  no  power  to 
digest  proteins  until  it  has  been  "  activated  "  by 
the  secretion  of  the  lining  wall  of  the  intestine,  a 
secretion  induced  by  the  flow  of  acid  liquid  from 
the  stomach.  The  action  is  due  to  the  presence  of 
a  complementary  body,  enterokinase,  which  ap- 
parently combines  with  the  zymogen  (or  enzyme- 
former),  called  trypsinogen,  to  form  trypsin,  the 
proteid-cleaving  enzyme  of  the  pancreatic  secretion. 
Similarly  it  has  been  discovered  that  the  muscles 
cannot   reduce   the  glycogen  which  is  necessary  as 


86  GENERAL   BIOLOGY 

a  source  of  their  activity,  without  the  presence  of  an 
activating  substance,  also  formed  in  the  pancreas 
and  transported  to  the  muscles  in  the  blood.  It 
seems  likely  that  the  majority  of  enzymes  are  thus 
compounded  of  a  zymogen,  secreted  in  the  cells  in 
the  form  of  granules,  and  an  activator  with  which 
it  must  unite  before  becoming  capable  of  its  specific 
action.  In  the  stomach  of  warm-blooded  animals 
the  activator  of  the  pepsin  is  hydrochloric  acid, 
which  is  only  excreted  under  the  stimulus  of  the 
presence  of  food.  But  there  seems  to  be  also  an 
antipepsin  formed,  which  neutralizes  the  enzyme 
and  prevents  self-digestion. 

Internal  Secretions  and  Hormones.  —  The  cells 
and  tissues  of  an  organism,  either  singly  or  grouped 
in  glands,  produce  a  variety  of  secretions,  such  as 
starch,  cellulose,  egg-yolk,  silica  crystals,  lime, 
mucus,  zymogen  granules,  etc.  These  products 
are  usually  visible,  and,  being  often  extruded  from 
the  cells  in  which  they  are  formed,  they  are  spoken 
of  as  external  secretions.  By  means  of  experi- 
ment it  has  been  demonstrated  that  in  addition  to 
the  external  secretion  of  certain  organs  there  is 
also  an  external  secretion,  so  called,  which,  instead 
of  collecting  in  ducts  and  being  thus  transported 
away  from  its  source  of  origin,  passes  directly  into 
the  blood  stream.  Many  "  ductless  "  glands,  such  as 
the  thyroid,  have  a  large  blood  supply,  which  takes 
up  such  an  internal  secretion,  but  other  glands,  such 
as  the  pancreas,  in  addition  to  the  evident  external 


METABOLISM  87 

secretion  (the  "pancreatic  juice,"  with  its  various 
digestive  enzymes),  have  been  shown  by  experiment 
to  develop  important  internal  secretions  as  well. 
Thus  it  has  been  found  that  total  extirpation  of 
the  pancreas  produces  the  unexpected  result  of 
inaugurating  glycosuria  (diabetes),  —  a  condition 
in  which  the  kidneys  constantly  eliminate  sugar 
from  the  blood.  If,  however,  only  a  small  portion 
of  pancreatic  tissue  be  left,  no  diabetes  or  only  a 
very  mild  form  results.  It  is  evident  that  the  exist- 
ence of  this  sort  of  diabetes  is  dependent  upon  the 
absence  of  pancreatic  tissue.  Even  when  the  pan- 
creas has  been  removed,  if  a  small  part  be  grafted 
in  w^here  the  blood  can  come  in  contact  with  it,  no 
diabetes  follows,  a  result  that  indicates  not  only 
that  the  ordinary  intestinal  secretion  has  nothing  to 
do  with  it,  but  also  that  it  is  necessary  for  the  blood 
to  flow  in  contact  with  the  pancreatic  tissue.  For 
this  and  other  reasons,  it  has  been  concluded  that 
the  pancreas  supplies  to  the  circulating  blood  an 
important  internal  secretion,  which,  in  some  way/ 
at  present  unknown,  controls  the  utilization  of  sugar 
in  the  animal  organism,  and  in  the  absence  of 
which  this  sugar  passes  out  of  the  body  unchanged. 
The  adrenals,  ductless  glands  attached  to  the 
kidneys,  have  been  shown,  in  much  the  same  way, 
to  produce  a  substance,  the  presence  of  which  in 
the  blood  rapidly  increases  the  blood  pressure  and 
produces  a  strong  contraction  of  the  peripheral 
blood-vessels.  This  substance  has  recently  been 
isolated  from  the  extract  of  the  gland,  and  is  found 


88  GENERAL  BIOLOGY 

to  be  a  white,  crystalline,  somewhat  bitter  powder, 
to  which  the  name  adrenalin  has  been  given.  Prac- 
tical use  of  this  substance  has  been  made  in  sur- 
gery. By  its  application  small  hemorrhages  may  be 
entirely  done  away  with,  particularly  in  delicate 
operations  on  the  nose  and  eye.  There  has  been 
synthesized  recently  a  substance  similar  to  adrenalin, 
if  not  identical  with  it,  which  produces  the  same 
effect. 

Another  ductless  gland,  the  thyroid,  by  its  se- 
cretion, influences  the  normal  phenomena  of  differ- 
entiative  growth  in  the  higher  animals.  When  the 
thyroid  is  diseased,  the  whole  system  is  affected. 
In  extreme  cases  degenerative  conditions  known 
as  "  cretinism  "  and  myxedema  result.  If  the  gland 
be  extirpated  in  a  very  young  animal,  death  in- 
evitably follows,  but  if  small  pieces  be  introduced 
elsewhere  in  the  body  by  grafting,  or  especially  if 
an  extract  of  the  gland  be  fed,  the  evil  results  dis- 
appear, or  are  greatly  mitigated.  The  extract  has 
been  shown  to  owe  its  efficacy  to  the  presence  therein 
of  a  chemical  compound  (thyroiodin)  containing  a 
high  percentage  of  iodine.'  Another  somewhat  sim- 
ilar ductless  gland,  the  thymus,  is  also  found  in  the 
throat.  If  tadpoles  be  fed  thyroid,  their  metamor- 
phosis is  greatly  hastened,  and  they  turn  into  tiny 
frogs  before  they  have  had  time  to  grow  to  normal 
size.  On  the  other  hand,  if  fed  thymus,  differen- 
tiation is  inhibited  and  growth  accelerated,  with  the 
result  that  they  grow  into  large  tadpoles,  but  do  not 
metamorphose  at  all. 


METABOLISM  89 

The  flow  of  half-digested  food  into  the  intestine 
mixed  with  hydrochloric  acid  (chyme)  that  is  poured 
stimulates  the  cells  of  the  lining  membrane  of  the 
latter  to  produce  a  substance  called  secretin,  which, 
passing  into  the  blood,  is  carried  to  the  pancreas. 
Here  it  stimulates  the  excretion  of  pancreatic  juice, 
which  flows  out  into  the  intestine,  apparently  in 
direct  response  to  the  inflow  of  food,  but  really  in 
response  to  another  sort  of  stimulus.  A  chemical 
excitant  like  secretin,  or  thyroiodin,  or  the  other 
products  just  described,  has  been  called  a  hormone^ 
and  it  is  likely  that  the  number  and  importance  of 
such  substances  will  be  greatly  increased  by  further 
investigation.  They  are  the  products  of  proto- 
plasmic activity,  like  the  zymogens,  but,  unlike 
them,  they  are  not  destroyed  by  boiling,  even  in 
hydrochloric  acid. 


CHAPTER  IV 
GROWTH 

In  all  normal  plants  and  animals,  anabolism 
nearly  always  tends  to  exceed  katabolism,  with  a 
consequent  increase  in  the  bulk  of  the  living  sub- 
stance. When  this  increase  in  volume  is  permanent, 
we  call  it  growth.  Such  changes  are  to  be  distin- 
guished from  temporary  changes  in  size  or  form  due 
to  the  rapid  imbibition  of  water  or  the  evolution  of 
gases.  They  are  also  to  be  distinguished  from  dif- 
ferentiating changes  such  as  occur  in  development. 
The  latter  may  or  may  not  be  accompanied  by  the 
increase  in  mass  called  growth.  In  plants,  growth 
is  a  phenomenon  which  generally  continues  as 
long  as  the  organism  lives.  In  animals,  it  is  a  special 
feature  of  the  earlier  period  of  the  organism's  life, 
and  then  usually  comes  to  an  end.  At  that  time 
a  balance  of  metabolism  is  struck,  after  which  the 
energies  of  the  organism  are  directed,  not  toward 
increase  in  size,  but  toward  reproduction.  For  this 
reason  the  period  of  greatest  growth  is  coincident 
with  immaturity.  The  enormous  disproportion  in 
the  amount  of  growth  in  the  earliest  stages  of  exist- 
ence is  illustrated  by  some  calculations  of  Professor 
Hertwig.  He  estimates  the  volume  "of  the  human 
ovum  at  .004  cubic  millimeter,  whereas  that  of  the 

90 


GROWTH  91 

child  at  birth  is  from  three  milUon  to  four  million 
cubic  millimeters,  an  increase  of  one  billion  times. 
Yet  from  the  first  year  to  the  twentieth  the  ratio  of 
increase  is  figured  at  only  one  to  sixteen. 

Since  the  organism  is  composed  of  cells,  it  is  obvious 
that  to  accomplish  this  growth  the  cells  themselves 
must  increase  either  in  size  or  in  number.  It  was 
early  discovered  that  both  these  changes  take  place, 
and  that  the  latter  seems  to  be  consequent  upon  the 
former.  We  have  seen  that  the  nucleus  "  domi- 
nates "  the  rest  of  the  cell,  as  it  were,  and  that  with- 
out the  presence  of  a  small  portion  of  nuclear  matter 
the  normal  changes  of  metabolism  in  the  cytoplasm 
cannot  go  on.  This  influence  of  the  nucleus  appears 
to  have  rather  narrow  limits,  and  if  the  cell  gets  to 
be  too  large,  portions  of  it  may  get  out  of  the  range 
of  the  nuclear  influence.  Sometimes  this  is  avoided 
by  the  fragmenting  of  the  nucleus,  the  parts  being 
distributed  about  the  cell ;  but  this  occurs  in  only 
a  few  kinds  of  cells.  Normally,  when  the  bulk  of  the 
cell  has  increased  by  growth  to  the  natural  limit,  the 
nucleus  divides  into  two  halves  that  move  apart  and 
divide  the  original  cell  between  them,  thus  making 
two  new  cells,  separated  by  a  newly  formed  cell- 
wall.  Occasionally  the  cell- wall  does  not  form,  in 
which  case  we  have  a  syncytium  resulting.  When 
the  two  daughter-cells  have  grown  to  the  size  of  the 
original  mother-cell,  the  process  is  repeated,  and  so 
on,  the  number  of  cells  in  a  given  tissue  increasing 
with  ^:he  growth  of  the  tissue,  but  the  average  size 
of  the  cells  themselves  remaining  nearly  constant. 


92  GENERAL  BIOLOGY 

Since  the  cell  gets  its  food  by  absorption,  it  is 
also  of  advantage  to  divide  in  this  way  in  order 
to  increase  the  absorptive  surface;  for  whereas 
solids  vary  as  the  cubes  of  a  dimension,  surfaces 
vary  only  as  the  squares  of  a  dimension.  To  use  a 
concrete  illustration,  the  combined  surface  of  two 
halves  of  an  orange  cut  in  two  in  the  middle  is 
greater  than  that  of  the  original  orange  by  just  the 
added  areas  of  each  cut  surface,  the  cubic  content 
remaining  the  same. 

Mitosis.  —  The  direct  cell  division  just  described, 
in  which  the  nucleus  simply  cuts  in  two  and  the  two 
halves  move  apart  while  the  cytoplasm  cleaves 
between  them  to  form  two  new  cells,  is  sometimes 
observed,  but  is  by  no  means  the  usual  method.  It 
will  be  remembered  that  the  content  of  a  cell  is 
normally  very  heterogeneous,  so  that  a  division 
plane  passed  through  the  middle  would  result  in 
producing  two  dissimilar  halves,  and  if  this  were 
repeated  a  number  of  times,  the  various  elements  of 
nucleus  and  cytoplasm  being  segregated  each  time, 
the  resulting  cells  would  soon  lose  all  real  resemblance 
to  one  another,  and  the  tissue  which  they  compose 
would  lose  its  homogeneous  character.  We  know 
that  this  does  not  happen.  Instead  of  this  direct 
method  of  division  we  find  that  another  sort  of  cell 
cleavage  usually  takes  place,  to  which  the  name 
of  mitosis  ^  has  been  given.  This  process  is  an 
extremely  complicated    one,   and    it    will    be  -more 

^  The  direct  cell  division  just  described  is  called  amitosis. 


GROWTH 


93 


readily  understood  if  we  preface  its  description  by  a 
simple  illustration.  Suppose  that  we  have  a  small 
box  full  of  marbles  of  different  kinds  and  sizes, 
some  glass,  some  agate,  some  porcelain,  etc.  By 
pushing  down  a  dividing  partition  exactly  in  the 
middle  we  could  divide  the  box  into  two  halves,  the 
cubic  content  of  each  of  which  would  be  the  same. 
It  will  be  seen  that  only  by  the  rarest  accident  would 
the  various  marbles  be  so  distributed  through  the 
box  before  we  divided  it  that  the  actual  contents  of 
each  half  space  would  be  identical,  and  then  only  if 


Fig.  32.  —  Four  stages  in  the  direct  (amitotic)  division  of  the  follicle  cells 
of  the  cricket's  egg.  —  (From  Dahlgren  and  Kepner.) 


there  were  no  odd  marbles  in  the  original  box.  Sup- 
pose, however,  that  to  begin  with  we  should  exactly 
divide  each  and  every  marble  in  two  and  put  one 
half  on  one  side  of  the  partition  and  the  other  half 
on  the  other  side.  The  result  would  be  that  the  two 
halves  of  the  divided  box  would  be  not  only  equal 
in  size,  which  they  were  before,  but  also  identical 
in  contents.  If  we  could  endow  box  and  contents 
with  the  power  of  growth,  we  see  that  when  both 
should .  have  doubled  in  size,  we  would  have  two 
boxes  of  marbles  each  similar  to  the  original  box. 


94  GENERAL  BIOLOGY 

Something  like  this  takes  place  in  the  process  of 
indirect  cell  division,  at  least  so  far  as  the  nucleus  is 
concerned. 

The  greater  part  of  cell  life  is  passed  in  the  so- 
called  "resting  stage,"  in  which  the  chromatin  sub- 
stance is  scattered  through  the  nucleus  apparently 
in  the  form  of  granules.  When  a  cell  is  preparing  to 
divide,  these  granules  begin  to  aggregate  and  fuse 
together  (or  at  least  appear  to  do  so,  though  it  is 
claimed  by  some  that  each  granule  retains  its  in- 
dividuality) .  By  such  an  aggregation  the  chromatin 
assumes  the  form  of  a  tangled  thread  or  skein. ^  This 
later  segments  into  a  number  of  separate  bodies  to 
which  the  name  chromosome  is  given.  The  number 
of  such  chromosomes,  which  varies  from  one  or  two 
to  a  hundred  or  more  in  each  cell,  is  normally  always 
constant  in  any  one  species.  Sometimes  they  even 
reveal  individuality  in  relative  size  and  shape.  At 
the  same  time  that  these  nuclear  changes  are  taking 
place  there  may  be  observed  in  the  cytoplasm  a 
tiny  dot  surrounded  by  a  mass  of  radiations.  This 
structure  is  appropriately  termed  the  aster  and  the 
central  body  the  centrosome.  This  appears  to  de- 
velop about  a  central  granule,  the  centriole.  The 
centrosome  divides  and  the  two  halves  move  apart 
about  the  nucleus,  each  apparently  carrying  a  por- 
tion of  the  aster  with  it,  until  they  have  placed 
themselves  on  opposite  sides  of  the  nucleus.  They 
are  connected  with  each  other  by  the  rays  of  the 
divided  aster  so  as  to  produce  what,  from  its  appear- 

^  Whence  the  name  mitosis,  from  fxiTos. 


// 


Fig.  33.  —  Diagrams  illustrating  mitotic  cell  division  :  A,  resting  cell ; 
B,  prophase  showing  spireme  and  nucleolus  within  the  nucleus  and  the 
formation  of  spindle  and  asters  (a)  ;  C,  later  prophase  showing  dis- 
integration of  nuclear  membrane  and  breaking  up  of  spireme  into 
chromosomes  ;  D,  end  of  prophases,  showing  complete  spindle  and  asters 
with  chromosomes  in  the  equatorial  plate  (ep)  ;  E,  metaphase  —  each 
chromosome  splits  in  two ;  F,  anaphase  —  the  chromosomes  move 
toward  the  asters ;  if,  interzonqj  fibers  ;  G,  telopljase  —  showing  recon- 
struction of  nuclei  ;  H,  later  telophase,  showing  division  of  the  cell  into 
two.  —  (From  Hegner,  after  Wilson.) 


ance,  is  called  a  spindle.  The  nuclear  membrane 
having  broken  down  in  the  meantime,  these  rays 
penetrate   the  nuclear   area   and   appear  to   fasten 


96  GENERAL  BIOLOGY 

themselves  to  the  chromosomes.  Through  the  pull 
exerted  by  the  contraction  of  these  fibers  the  chro- 
mosomes are  swung  into  an  equatorial  plane.  Not 
all  the  fibers  radiating  from  the  centrosomes  go  to 
make  up  the  spindle  proper;  other  radiations  at 
the  sides  —  the  so-called  mantle  fibers  —  attach 
themselves  to  the  cell-wall  or  lose  themselves  in 
the  cytoplasm. 

The  next  step  consists  in  a  longitudinal  splitting  of 
each  chromosome  and  the  shortening  of  the  spindle 
fibers  attached  to  each  side  so  that  the  two  half 
chromosomes  separate.  That  this  splitting  of  the 
chromosomes  is  only  accompanied  by  and  not  caused 
by  the  fastening  on  them  of  the  spindle  fibers  is  evi- 
denced by  the  fact  that  in  many  instances  the  chro- 
matin aggregate  in  the  skein  or  "  spireme "  stage 
splits  precociously  before  the  spindle  has  formed  or 
the  spindle  fibers  have  become  attached .  The  divided 
chromosomes  move  toward  the  poles  of  the  spindle, 
(or  are  dragged  toward  them),  and  in  the  interval 
between  may  be  seen  fibrils  of  the  spindle  in  the 
midst  of  which  a  row  of  granules  often  appears, 
foreshadowing  the  formation  of  the  new  cell- wall. 
When  the  chromosomes  have  moved  to  each  pole  of 
the  spindle,  the  reverse  of  the  preparatory?^  changes 
previously  described  begins  to  take  place,  and  the 
individual  chromosomes  fuse  together  into  a  spireme 
which  eventually  breaks  up  into  a  mass  of  granules 
characteristic  of  the  original  *'  resting  stage." 

The  stages  involved  in  mitotic  cell  division  may  be 
made  clearer  by  a  diagrammatic  summary. 


GROWTH  97 

Nucleus  Cytoplasm 

1.  Formation  of  spireme.  Appearance  of  centrosome  and 

aster. 

2.  Segregation     of     chromo-   Division     of     centrosome    and 

somes.  aster. 

3.  Breaking   down    of    mem-   Moving   apart  of   centrosomes 

brane.  to    opposite    poles;    attach- 

4.  Formation     of     equatorial       ment    of     spindle    fibers    to 

plate.  chromosomes. 

5.  Splitting   of  clfromosomes. 

6.  Separation     of     daughter     Contraction  of  spindle  fibers. 

chromosomes. 

7.  Re-formation  of  spireme.     Disappearance    of    astral    rays 

and  centrosomes. 

8.  Disintegration   of    spireme   Formation  of  new  cell-wall. 

into   granules  of  resting 
stage. 

Stages  1-4  are  sometimes  called  "  prophases," 
stage  5  "  metaphase,"  and  stages  6-8  "  anaphases." 

Such  is  the  orderly  series  of  changes  undergone 
in  all  cells  that  divide  indirectly.  The  synchrony 
of  movements  in  nucleus  and  cytoplasm  is  not  always 
just  the  same  as  indicated  above,  but  the  modifications 
are  not  fundamental,  usually  consisting  in  a  hastening 
or  a  delaying  of  changes  on  either  one  side  or  the 
other,  and  the  end-result  is  always  the  same. 

Abnormal  Mitosis.  —  Under  certain  conditions, 
such  as  the  division  of  an  egg-cell  fertilized  by  more 
than  one  sperm,  or  in  degenerative  growths  as  in 
cancer  and  tumors,  often  three  or  more  centrosomes 
and    spindles    appear,    and    as    a    consequence    the 


98 


GENERAL  BIOLOGY 


chromosomes  are  laid  hold  of  on  all  sides,  "  multi- 
polar "  spindles  being  formed.  These  phenomena 
indicate  that  the  spindle  is  a  complicated  piece  of 
mechanism,  which  like  all  machines  may  "  go  wrong." 


Nature  of  the  Centrosome.  —  The  centrosome, 
from   the  leading   part  it   appears  to   play  in  cell 

division,  has  been  the 
object  of  very  careful 
research.  It  has  been 
held  by  some  that  it 
is  a  permanent  organ 
of  the  cell,  but  this 
seems  not  to  be  the 
case.  Not  only  is  it 
normally  absent  in 
many  dividing  cells, 
but   there  is  no  ques- 

FiG.     34.  —  Abnormal     mitoses,  tiou       but      that      it      is 

Four-poled    spindle   in    a    developing  „               , 

sea-urchin's  egg  after  poisoning  with  lOrmeCl     aUCW     in     SUC- 

quinine       The   spindle   x-y   lacks   its  cccdiug    Cell     divisioUS, 

share   of    the  chromosomes.  —  (Hert-  '^ 

wig.)  •  most  often  in  the  cyto- 

plasm. Centrosomes, 
moreover,  have  been  caused  to  develop  by  chemical 
reagents  in  enucleated  fragments  of  unfertilized  eggs 
after  the  true  egg  centrosome  and  spindle  has 
developed  in  another  part  of  the  same  egg. 

The  mechanical  basis  of  the  mitotic  spindle  has 
been  difficult  to  get  at,  because  practically  the  only 
means  of  studying  cell  structure  is  by  killing,  fixing, 
hardening,  sectioning,  and  staining  cells  and  tissues. 


GROWTH  99 

and  it  is  sometimes  hard  to  decide  whether  all  that 
we  observe  existed  in  the  living  cell  or  are  artifacts 
produced  by  our  manipulations.  The  spindle  is 
the  physical  expression  of  forces  operating  within 
the  cell,  but  whether  as  cause  or  as  effect  is  not  easy 
to  say,  and  there  have  not  been  wanting  theorists 
who  interpreted  the  phenomena  from  either  point 
of  view.  Comparisons  have  been  instituted  be- 
tween the  spindle  and  the  lines  of  force  depicted 
when  a  horseshoe  magnet  is  held  under  a  layer  of 
iron  filings,  and  the  claim  has  been  made  that  the 
spindle  we  see  is  but  the  arrangement  of  the  particles 
of  protoplasm  in  accordance  with  lines  of  a  force, 
itself  as  invisible  as  magnetism.  On  the  other 
hand,  dividing  cells  have  been  dissected  in  the 
living  condition,  and  it  has  been  amply  demonstrated 
that  the  spindle  and  the  asters  are  real  and  tangible 
things,  and  that  the  separation  of  the  chromosomes 
is  accompanied  by  the  contraction  of  these  threads. 
It  has  even  been  suggested  that  the  spindle  fibers 
are  the  result  of  chemical  changes  in  the  semiliquid 
cytoplasm  analogous  to  the  formation  of  the  ropy 
fibrin  in  the  clotting  of  the  fluid  plasma  of  the 
blood. 

Cell  multiplication  is  to  be  looked  upon,  not  as  an 
end  in  itself,  but  as  a  readjustment  of  the  physico- 
chemical  relation  of  two  sorts  of  protoplasm  con- 
tained in  nucleus  and  cytoplasm,  following  an 
increase  in  substance.  Considering  the  tissue  as  a 
whole,  this  growth  is  a  continuous  process ;  from  the 
standpoint  of  the  cells  as  physiological  units  it  is 


100  GENERAL  BIOLOGY 

discontinuous.     But  the  discontinuity  is  thus  only 

apparent,  not  real. 
Indeed  there  is  always 
retained,  not  only  a 
physiological,  but  also 
a  certain  physical  con- 
tinuity between  the 
cells. 


Influence  of  Exter- 
nal Conditions  on 
Growth 

Light.  —  One  of  the 
factors  of  most  signifi- 
cance in  the  normal 
growth  of  animals  and 
plants  is  that  of  light. 
It  makes  a  great  differ- 
ence, however,  whether 
this  light  is  diffuse  or 
whether  it  is  the  direct 
sunlight.  Sunlight 
completely  inhibits  the 
growth  of  many  Pro- 
tista and  lower  plants 
(bacteria,  molds,  etc.) 
and  destroys  the  organ- 

FiG.   35. -Peas    grown    6    days    in  ISmS.       For    this   reaSOU 

darkness  (a),  in  about  ^  light  (6),  and  sunlight    is    OUC    of    the 

open  in  greenhouse  (c).     (From   Dug-  rv        •                 •!•    • 

gar's  Plant  Physiology.)  mOSt  etteCtlVe  SterillZmg 


GROWTH  m 

agents  known.  The  majority  of  animals  and  plants, 
however,  require  an  abundance  of  light  for  normal 
growth  and  development.  Seedlings  grown  in  the 
dark  and  those  grown  in  diffuse  light  present  a 
striking  contrast.  The  former  are  "  spindling," 
weak  and  pale,  and  always  much  larger  than  those 
grown  in  the  light.  Even  diffuse  light  thus  seems 
to  have  a  retarding  effect  on  growth  as  such,  although 
necessary  for  the  normal  development  of  the  plant. 
The  difference  in  size  between  plants  grown  in  the 
dark  and  those  grown  in  the  light  is  perhaps  due  to 
the  loss  of  water  that  takes  place  in  the  light,  with 
a  consequent  concentration  of  protoplasm.  This 
idea  is  supported  by  the  fact  that  aquatic  plants  and 
animals  grow  faster  in  the  light  than  in  the  dark, 
owing  doubtless  to  the  fact  that  such  a  loss  of  water 
does  not  occur. 

The  white  light  of  the  sun  may  be  broken  up  into 
the  components  of  the  spectrum,  and  it  has  been  dis- 
covered that  the  various-colored  rays  of  the  spectrum 
have  very  different  effects  upon  growth.  The  actinic 
(ultra-violet)  rays  are  the  most  active,  and  it  is 
probably  their  presence  that  makes  direct  sunlight 
so  destructive  to  living  matter.  The  action  of  the 
other  rays  varies,  but  in  some  plants  there  is  a  pro- 
gressive inhibition  of  growth  from  the  red  end  of  the 
spectrum  to  the  other. 

Temperature.  —  Various  different  organisms  are  to 
be  found  thriving  in  all  extremes  of  heat  and  cold 
between  0°  C.  and  a  very  Httle  below  the   boiling 


10^ 


GENEKAL  BIOLOGY 


point.     In  general,  however,  growth  is  accelerated 
both  in  plants  and  animals  by  a  slight  increase  of  the 


eO°FAHR. 

MEAN  TEMPERATURE 
66°FAHR.                    53^FAHR. 

51°FAaR. 

1848 
MARCH  11TH 

€f 

m 

<» 

0 

20TH 

^Q 

® 

d 

& 

23D 

>* 

'25TH 

^*Q 

e 

@ 

27TH 

J 

28TH 

^ 

3181 

3Q 

«o 

APRIL  4TH 

> 

flTH 

-^ 

^ 

10TM 

j0f^ 

MAY  22D 

% 

-i#* 

.^ 

>• 

AUG.  18TH 

■*- 

28th 

X 

OCT.  31  ST 

^ 

Fig.  36.  —  Chart  showing  the  correlation  between  the  stage  of  de- 
velopment of  the  frog  on  successive  days  and  the  temperature  at  which 
it  has  developed.  —  (From  Davenport,  after  Higgenbottam.) 

normal  temperature.     This  is  well  illustrated  by  the 
accompanying  chart  (fig.  36). 

Effect  of  Chemical  Agents.  —  Since  growth  can 
take  place  only  at  the  expense  of  food  substances 


GROWTH  103 

which  are  elaborated  into  protoplasm,  it  is,  of  course, 
obvious  that  the  chemical  nature  of  the  available 
food  supply  will  have  a  marked  influence  upon  the 
growth  of  both  plants  and  animals.  Careful  ex- 
periments seem  to  show  that  growth  is  advanced 
most  in  the  presence  of  abundant  nitrggenQus_com-_ 
poinds,  next  in  the  presence  of  fatsj  and  leastjiLlhaL 
of  carbohydrates.  Growth  may  also  be  greatly 
accelerated  by  various  chemical  stimuli.  Thus  it 
has  been  found  that  many  poisons,  deadly  to  the 
organism  when  in  any  considerable  concentration, 
yet  stimulate  growth  when  in  extreme  dilution. 
Mercuric  chloride,  strychnine,  cocaine,  ether,  and 
many  other  poisons  act  in  this  way. 


CHAPTER  V 

TISSUE-DIFFERENTIATION    FOR     SPECIFIC 
FUNCTIONS 

I.  Differentiation  in  Animals 

In  the  Protista  cellular  differentiation  is  identical 
with  differentiation  of  the  organism.  It  would  be 
the  same,  so  far  as  Paramecium  is  concerned,  if  each 
cilium  were  a  tiny  vibratile  cell  instead  of  being,  as 
most  biologists  believe,  a  minute  portion  of  the  one 
cell  composing  the  body  of  the  animal.  We  then 
might  speak  of  these  locomotor  organs  as  the 
"  ciliary  system."  In  the  many-celled  animals  and 
plants  the  organism  also  acts  as  a  unit,  and  the 
differentiation  of  cells  for  the  better  performance  of 
specific  functions  such  as  we  have  traced  in  the 
previous  chapter  is  carried  out  apparently  with  ref- 
erence only  to  the  needs  of  the  whole.  The  struc- 
ture of  the  higher  animals  is  so  complex  and  the  vari- 
ous systems  so  specialized  that,  in  studying  them, 
this  point  is  often  lost  sight  of. 

Alimentary  System.  —  The  taking  in  of  food  is 
the  most  necessary  function  of  living  things,  and  only 
those  forms,  like  the  tapeworm,  which  have  become 
absolved  from  the  responsibility  of  seeking  food  on 
their  own  account  are  without  a  special  apparatus 
for  seizing,  storing,  and  reducing  foods.     In  practi- 

104 


TISSUE-DIFFERENTIATION 


105 


cally  all  animals  there  is  a  central  tubular  cavity  into 
which  the  food  is  taken  to  be  dissolved  or  digested 
by  the  secretions  elaborated  for  this  purpose.     In  the 


Fig.  37.  —  Diagrams  of  various  types  of  digestive  systems  :  A,  Hydra 
in  which  there  is  no  distinction  between  body  cavity  and  digestive 
cavity ;  B,  a  (polyclad)  flatworm  in  which  there  is  no  body  cavity  and 
but  one  opening  to  the  many-branched  digestive  cavity ;  D,  anterior  end 
of  earthworm  ;  E,  section  of  snail  ;  F,  alimentary  canal  of  human  being  ; 
m,  mouth  ;  ph,  pharynx  ;  en,  enteron  or  digestive  cavity  ;  st,  stomach  ; 
cr,  crop ;  g,  gizzard ;  i,  intestine ;  s.i.,  small  intestine ;  c,  colon ; 
r,  rectum. 


simplest  of  the  Metazoa  {Ccelenterata  and  "  flat- 
worms  ")  this  sac  has  but  one  external  opening 
through  which  all  the  food  is  taken  in  and  from  which 
all  the  non-utilizable  parts  are  ejected  (fig.  37).     In 


106  GENERAL  BIOLOGY 

other  animals  the  tube  is  open  at  each  end  and  the 
food  passes  slowly  from  mouth  to  vent.  A  few  forms, 
such  as  the  snails,  have  both  openings  close  together. 
Some  animals,  such  as  the  pigeon,  or  the  deer,  are 
compelled  to  seek  their  food  in  danger  of  attack  and 
have  developed  the  habit  of  taking  in  a  quantity  of 
food  to  be  stored  in  a  crop  or  similar  sac  to  be  di- 
gested at  leisure  in  safer  surroundings.  Probably 
the  stomach,  which  is  found  in  one  form  or  another 
in  most  animals,  functions  primarily  as  a  storage 
chamber. 

The  differentiation  of  the  alimentary  canal  is 
usually  specially  related  to  the  habits  of  the  animals. 
Thus,  meat  eaters  require  far  less  bulk  to  obtain  the 
same  amount  of  food  substance  than  do  vegetable 
eaters,  and  we  find  the  alimentary  canal  in  the  latter 
much  larger  and  longer.  In  the  cat  the  alimentary 
canal  is  but  three  times  the  body-length,  whereas  in 
a  sheep  it  may  be  as  much  as  twenty-eight  times  as 
long.  In  order  to  increase  the  absorptive  surface 
and  hasten  the  taking  up  of  large  quantities  of 
digested  food  the  inner  surface  of  the  alimentary 
canal  is  often  thrown  into  folds  and  wrinkles  which 
greatly  increase  the  superficial  area  without  increas- 
ing the  size  of  the  tube.  Thus  in  the  earthworm 
there  is  developed  the  so-called  typhlosole,  which  is  a 
fold  of  the  dorsal  wall  of  the  intestine  that  hangs 
pendent  within  the  cavity  of -the  tube.  In  many 
fishes  the  same  end  is  attained  through  the  develop- 
ment of  the  spiral  valve,  which  is  a  fold  of  the  inner 
lining  of  the  intestine  developed  in  the  form  of   a 


TISSUE-DIFFERENTIATION  107 

screw  and  largely  increasing  the  absorptive  surface. 
In  mammals  the  whole  inner  surface  of  the  intestine 
is  lined  with  innumerable  finger-shaped  processes 
called  villi  that  project  into  the  intestinal  cavity  and 
enormously  increase  the  absorptive  area. 

Sensory  Organs.  —  Since  food  must  usually  be 
captured  before  being  eaten,  we  find  that  in  the  great 
majority  of  animals  (the  "higher"  ones  particularly) 
all  the  sense  organs  available,  whether  of  seeing  or 
touching,  smelling  or  tasting,  are  concentrated  in 
close  proximity  to  the  mouth.  This  specialization  of 
organs  in  one  portion  of  the  body  has  brought  about 
a  sharp  differentiation  of  that  part  from  the  rest  of 
the  body,  a  cephalization  or  "  head-specialization." 
Sensory  organs  in  the  beginning  were  probably 
diffusely  scattered  over  the  body  as  they  are  now  in 
the  simplest  types,  and  the  concentration  of  this 
system  in  one  locality,  i.e.  the  head,  has  made  neces- 
sary a  means  of  communication  between  such  centers 
and  more  remote  parts.  Thus  we  find  a  system  of 
nerves  connecting  the  sensory  centers  with  other 
parts  of  the  body,  particularly  the  muscles,  and 
affording  paths  for  stimulating  "  impulses  "  which 
result  in  coordinating  movements  and  thus  unifying 
action. 

Concentrations  of  nervous  elements  into  centers  of 
this  sort  are  termed  ganglia  (singular,  ganglion). 
The  ganglia  are  larger  and  more  complex  in  the  region 
where  the  majority  of  sensory  impulses  arise,  that  is, 
the  head,  and  in  higher  forms  of  animal  life  these 


Full. 


One  hour  Twelve  Eighteen 

after  hours.  hours, 

changing. 
Immediately 
after 
pupation.         Fig.  38.  —  Changes  in  the    nervous 

system  of  a  butterfly,  illustrating 

mechanical  "  cephalization."  —  (From  Packard, 

after  Newport.) 

8  9  10 


Twenty- 


•^ 


l^^ 


'2lP 


Forty-eight 
hours. 


TISSUE-DIFFERENTIATION 


109 


nervous  centers  reach  a  large  size  and  a  very  high 
degree  of  complexity.  They  are  then  called  the  brain. 
But  most  animals  are  more  or  less  elongate,  and  this 
central  condensation  of  the  nervous  system  is  also 
distributed  along  the  longitudinal  axis.  In  verte- 
brates this  forms  the  spinal  cord,  which  is  a  thick- 
walled  hollow  tube.     In  invertebrates  the  central 


^^ 


^v  /  /  /  2-J. 


FiQ.  39.  —  Diagrammatic  comparison  between  the  vertebrate  type 
{A)  and  the  invertebrate  type  (5),  with  respect  to  the  relation  of  the 
nervous  system  to  the  alimentary  canal. 


nervous  system  is  stjid  and  often  double,  or  in  the 
form  of  a  ladder.  Iiweriebrates  the  central  nervous 
system  and  brain  is  constantly  dorsal  ^  to  the  alimen- 
tary tube ;  in  invertebrates,  with  few  exceptions,  the 
central  nervous  system  is  ventral  to  the  alimentary 
canal,  although  the  brain  is  always  dorsal.  Accord- 
ingly the  connections  between  the  brain  and  the  rest 
of  the  nerve  cord  in  invertebrates  have  to  go  around 

^  "  Dorsal "  and  "  ventral "  are  here  used  in  their  ordinary  though 
somewhat  inaccurate  meanings. 


110  GENERAL  BIOLOGY 

the  alimentary  canal  in  a  sort  of  ring  or  collar.     (See 
fig.  39.) 

Skeletal  Structures. — Endoskeleton. — In  the  Pro- 
tozoa the  cell-body  is  so  small  that  it  holds  together, 
as  a  rule,  of  its  own  viscosity.  Yet  even  here,  in 
those  groups  in  which  the  protoplasm  is  most  foamy 
and  watery,  we  find  a  skeleton  or  supporting  frame- 
work developed  which  gives  rigidity  and  form  to  the 
protoplasm  itself.  Sometimes,  as  in  the  sun-animal- 
cules (Heliozoa),  this  skeleton  attains  great  com- 
plexity. In  some  groups  of  Protozoa  it  is  composed 
of  silica,  in  others,  of  lime.  In  the  corals,  the  soft 
"  polyps  "  secrete  so  much  skeleton  that  the  animal 
portion  becomes  but  a  fraction  of  the  whole.  In  the 
sponges  the  skeleton  is  made  up  of  innumerable 
needles  or  spicules  of  lime  or  silica.  In  one  group  of 
sponges  the  skeleton  is  composed  of  a  network  of 
tough  fibers  of  peculiar  composition.  When  the 
protoplasmic  portion  of  the  sponge  has  been  soaked 
out,  this  skeleton  forms  the  "  bath-sponge "  of 
commerce. 

In  the  larger  animals  the  weight  of  the  body  would 
make  it  impossible  for  a  constant  form  to  be  main- 
tained were  it  not  for  the  fact  that  there  exists  an 
internal  framework  or  scaffolding  to  which  the  softer 
parts  are  attached.  In  the  vertebrates  this  is  usually 
made  up  of  mineral  elements,  the  bones,  although  in 
some  of  the  lower  fishes  the  skeleton  is  composed  of 
cartilage  (gristle)  instead  of  bone.  This  bony  frame- 
work may  be  divided  into  two  systems,  one  under- 


TISSUE-DIFFERENTIATION 


111 


lying  the  central  nervous  system  and  hence  con- 
stituting the  longitudinal  axis  of  the  animal  (axial 
skeleton),  and  the  other  giving  support  and  rigidity 
to  the  limbs  (appendicular  skeleton).     In  addition, 


Fig.  40.  —  Endoskeleton  of  man.  —  (From  Coleman.) 

the  brain  and  connected  sense  organs  are  surrounded 
and  protected  by  an  enveloping  skull,  to  which  is 
attached  the  bony  framework  of  the  jaws.  The  axial 
skeleton  is  divided  into  segments  called  vertebrae 
which  provide  the  proper  balance  between  flexibility 


112  GENERAL  BIOLOGY 

and  rigidity.  In  development,  this  "  backbone  "  is 
preceded  by  an  unjointed  rod  of  cartilage  called  the 
notochord,  which  is  ultimately  replaced  by  the  verte- 
brae. The  very  lowest  types,  however,  do  not 
develop  any  other  axial  skeleton  than  the  notochord. 
The  great  group  of  animals  possessing  this  structure, 
either  in  the  embryo  or  in  the  adult,  is  called  the 
Chordata.  Thus,  all  vertebrates  are  chor dates,  but 
some  chordates  are  not  vertebrates  (i.e.  have  no  true 
backbone) . 

Exoskeleton.  —  In  many  of  the  simpler  forms  of 
animal  life,  instead  of  an  internal  skeletal  frame- 
work upon  which  the  softer  parts  are  suspended,  there 
is  developed  a  thick  and  hard  external  skeleton,  which 
not  only  supports  the  internal  organs,  but  protects 
them  as  well.  Particularly  in  the  insects  and  the 
Crustacea  (crabs  and  their  relatives)  the  outer  skin 
secretes  a  very  tough,  hard  substance  called  chitin 
which  forms  a  sort  of  shell,  jointed  at  appropriate 
places  to  permit  of  free  movement.  The  animal  is 
thus  provided  with  a  semipermanent  suit  of  armor 
which,  being  non-elastic,  must  be  shed  periodically 
to  permit  of  body  growth.  Many  of  the  vertebrates 
that  do  not  require  an  exoskeleton  for  the  purpose  of 
body  support  are  provided  with  such  skeletal  struc- 
tures for  protection.  Thus  the  scales  of  fishes  and 
snakes  are  a  sort  of  very  flexible  coat  of  mail,  and 
the  feathers  of  birds  and  thick  hair  of  wild  animals 
belong  to  the  same  category.  In  the  turtles  the 
development  of  protective  armor  has  gone  on  to  such 


TISSUE-DIFFERENTIATION 


113 


an  extent  that  the  whole  body  is  inclosed  in  a  heavy 
bony  box.  In  the  mollusks  (shellfish,  snails,  etc.) 
this  sort  of  protective  structure  has  taken  a  different 
line  of  development  through  the  formation  of  a 
calcareous  shell.  This  is  often  complex  in  its  struc- 
ture, and  composed  of  several  layers.  It  is  usually 
developed  to  such  an  extent  that  the  weight  of 
mineral  covering  reduces  the  free  movement  of  the 
animal  to  a  minimum. 


Neural  Plate 


^^^^-—     I  vEpiderm 


>CuU5. 


Fig.  41.  —  Diagrammatic  transverse  section  through  the  shell  of  the 
tortoise.  On  the  right  side  the  bony  shields  have  been  removed.  The 
endoskeleton  is  cross-lined,  the  exoskeleton  is  dotted.  »Sp.C,  spinal 
chord  ;  Ca-p,  head  of  rib.  —  (From  Gadow.) 


Muscular  System.  —  The  tissues  involved  in  loco- 
motion and  other  movements  act  by  their  power  of 
contraction.  In  order  to  bring  about  orderly  move- 
ments they  must  be  attached  to  rigid  supports.  In 
the  vertebrates,  these  are  the  bones  of  the  skeleton, 
to  which  the  muscles  are  firmly  attached.  The 
bones  themselves  are  covered  with  a  tough  skin  of 


114 


GENERAL  BIOLOGY 


connective  tissue,  the  periosteum.  This  is  continu- 
ous with  a  dense  inelastic  tendon  that  spreads  over 
and  permeates  the  muscle  bundles,  forming  the 
perimysium.  Muscle  and  bone  are  thus  most 
intimately  bound  together  into  a  unit.  In  inverte- 
brates with  a  chitin- 
ous  exoskeleton  the 
muscles  are  usually 
attached  to  the  hard 
shell  or  to  internal 
plates  that  arise  from 
the  shell.  In  forms 
like  the  worms,  in 
which  the  whole  body 
contracts  strongly,  the 
muscles  are  to  be 
found  in  sheets,  usu- 
ally running  in  differ- 
ent directions,  and 
exerting  a  reciprocal 
action  on  one  another. 
In  the  very  lowest 
and  simplest  forms 
they  occur  as  isolated  fibers.  In  such  cases  they 
represent  the  first  step  in  the  specialization  of  the 
contractile  function  common  to  all  protoplasm. 


Fig.  42.  —  Diagrammatic  cross-sec- 
tion through  the  thorax  of  an  insect, 
showing  the  nature  of  the  musculature 
and  the  manner  of  the  insertion  of  the 
muscles  to  the  exoskeleton  :  ht,  heart ; 
n,  nerve  cord  ;  wg,  root  of  wing  ;  Uj, 
root  of  leg  ;  ap,  apodeme  or  chitinous 
brace ;  Im,  longitudinal  muscles  of  the 
"back  "  ;  m,  dorso-ventral  muscles  oper- 
ating wings  and  legs.  —  (After  Graber.) 


Circulatory  Systems.  —  In  most  of  the  lower 
animals,  particularly  those  protected  by  a  tough  or  a 
hard  outer  covering,  the  tissues  of  the  interior  are 
very  soft  and  loosely  held  together.     The  organs  are. 


TISSUE-DIFFERENTIATION  1 15 

as  it  were,  awash  in  the  body  fluids.     The  ahmentary 
canal  occupies  a  relatively  large  part  of  the  interior 
space,    and,    in   consequence,    the   digested   food   is 
absorbed  and  passed  on  by  diffusion  to  all  parts  of 
the  body.     Likewise  the  waste-products  of  metabol- 
ism find  their  way  without  difficulty  to  the  special 
organs  that  provide  for  their  elimination.     In  the 
larger  animals,  and  particularly  in  those  with  rela- 
tively solid    tissues,  with  a  great  development    of 
muscles  and  an  alimentary  canal  of  small  dimensions 
compared  with  the  whole  body  cavity,  it  Would  be 
impossible  for  the  digested  food  to  find  its  way  to 
the  places  where  it  is  most  needed,  particularly  to  the 
skeletal  muscles.     Appropriate  to  the  need  for  trans- 
porting such    substances    long    distances    from    the 
alimentary  canal  and  also  of  transporting  the  meta- 
bolic waste-products  from  the  seat  of  their  produc- 
tion to  the  excretory  organs,  there  is  a  system   of 
tubes,  the  circulatory  system,  which  performs  the 
same  function  carried  out  by  a  railroad  system  in  a 
thickly   settled  community;   that   is,    it   transports 
substances  from  the  region  where  they  are  produced 
to  the  region  where  they  are  utilized.     The  medium 
of  transportation  in  the  case  of  the  organism  is  the 
blood.     Thus,  not  only  is  the  digested  food  carried 
from  the  intestine  to  the  tissues  which  are  to  be  fed, 
or   to   organs   that   serve   as    storehouses,    but   the 
oxygen  taken  in  in   respiration  is   supplied  to  all 
the  tissues,  the  various  hormones  are  transported 
from  one  place  to  another,  and  the  waste  products  of 
katabolism  are  drained  off  to  the  excretory  organs  by 


116 


GENERAL  BIOLOGY 


the  same  route.  Such  a  system  in  its  most  elemen- 
tary form  may  be  found  in  the  insects.  Here  it 
consists  of  a  simple  dorsal  tube  open  at  both  ends, 
which  by  its  alternate  contractions  and  expansions 
keeps  the  body  fluids  "  stirred  up  "  and  provides  for 


Fig.  43.  —  Diagrams  of  the  circulation  of  the  frog  (^4)  and  of  the 
lobster  (B),  the  former  illustrating  a  closed  system,  the  second,  an 
"open"  system:  L,  left  auricle;  R,  right  auricle;  V,  ventricle;  1, 
arterial  branch  to  the  head  ;  2,  arterial  branch  to  the  rest  of  the  body ; 
3,  arterial  branch  to  the  lung  (P)  ;  4,  pulmonary  vein  returning  aerated 
blood  from  the  lung  to  the  left  auricle  ;  5  and  6,  venous  trunks  return- 
ing blood  from  the  head  and  body  to  the  right  auricle  ;  H,  heart  sending 
out  arterial  blood  both  to  the  anterior  and  to  the  posterior  part  of  the 
body ;  s,  sinuses,  in  which  the  blood  accumulates  to  be  returned  through 
the  gills  (G)  to  the  pericardial  chamber  (p.c),  whence  it  finds  its  way  to 
the  interior  of  the  heart  through  the  ostia  (o) . 

an  indefinite  circulation  of  substances.  In  the  sides 
of  the  tube  are  mouthlike  apertures  provided  with 
valves,  called  ostia.  Through  these  the  body  fluids 
can  enter  when  the  tube  expands,  but,  owing  to 
the  valves,  they  can  leave  only  through  the  open 
ends.  The  necessity  for  a  complicated  set  of  blood 
tubes  in  these  animals  is  obviated  by  the  develop- 


TISSUE-DIFFERENTIATION  117 

meivt  of  a  remarkable  system  of  air-tubes,  the 
trachea,  which  open  to  the  exterior  and,  permeating 
tlie  insect  body  in  every  direction,  permit  a  direct 
supply  of  atmospheric  oxygen  to  get  to  all  the  tissues. 
The  lobster  presents  a  next  higher  step  in  differ- 
entiation and  specialization.  Instead  of  the  elongate 
dorsal  tube  there  is  a  boxlike  heart,  likewise  pro- 
vided with  ostia,  but  opening  also  into  a  number  of 
blood  tubes  (arteries)  that  carry  the  bk)od  away  from 
the  heart.  These  arteries  divide  and  subdivide 
until  finally  they  empty  their  contents  into  the  open 
spaces  between  the  muscles  and  other  organs,  called 
sinuses.  From  these  spaces  the  blood  seeps  back 
and,  after  traversing  the  gills,  reenters  the  heart 
through  the  ostia.  Thus  a  circuit  is  completed, 
definite  through  part  of  its  extent  and  indefinite 
through  the  rest.  In  vertebrates  the  blood  not  only 
leaves  the  heart  by  way  of  arteries,  but  is  returned 
to  it  by  another  system  of  tubes  called  veins,  the 
two  systems  being  connected  by  much  finer  vessels 
called  capillaries.  Thus  the  entire  circuit  is  com- 
pleted within  definite  channels.  Such  a  system  is 
called  a  "  closed  "  system  in  contrast  to  the  "  open  " 
system  of  the  lobster. 

Excretory  Organs.  —  The  waste-products  men- 
tioned above  are  not  only  of  no  use  to  the  organism, 
but,  on  the  other  hand,  in  most  cases,  are  active 
poisons,  whose  ill  effects  are  evident  if  they  be  ever 
so  slightly  concentrated.  It  is  of  the  highest  impor- 
tance to  the  animal  that  these  substances  be  elimi- 


118 


GENERAL  BIOLOGY 


nated  as  soon  after  their  formation  as  possible.     Even 
in  the  least  differentiated  of  the  Protozoa   special 


Fig.  44.  —  Diagram  of  various  types  of  excretory  organs;  A,  Par- 
amecium with  two  pulsating  vacuoles,  the  upper  one  is  at  the  close  of 
cycle,  the  lower  one  at  the  beginning  ;  B,  flame-cell  of  a  "flatworm"  ; 
fl,  the  wisp  of  cilia,  the  vibration  of  which  urges  the  excreted  matter 
down  the  duct ;  n,  nucleus  ;  gr,  granules  of  waste  products  ;  C,  simple 
nephridium  of  Polygordius ;  d,  septum ;  st,  nephrostome  or  ciliated 
funnel  ;  op,  external  orifice ;  D,  nephridium  of  a  highly  specialized 
annelid  worm,  the  blood  vessels  in  black ;  E,  nephridial  element  of 
vertebrate  kidney ;  g,  glomerulus ;  cap,  capsule  of  urinary  tubule 
ciliated  in  its  lower  portion,  blood-vessels  in  black. 


structures  are  found  which  provide  for  such  elimina- 
tion.    In  such  organisms  there  is  usually  a  spot  on 


TISSUE-DIFFERENTIATION  119 

the  outer  margin  of  the  cell-body  toward  which  the 
dissolved  "  waste-products "  concentrate.  These 
forni  a  gradually  enlarging  bubble  which  finally 
ruptures  and  squeezes  its  contents  out  through  the 
cell-wall  to  the  exterior.  In  most  Protozoa  this 
excretory  function  is  definitely  localized,  and  the 
formation  and  extrusion  of  the  excreted  drop  of 
fluid  follows  a  regular  rhythm.  Such  an  organ  is 
called  a  "  contractile  vacuole."  With  the  enlarge- 
ment of  the  body  and  the  multiplication  of  the  cellu- 
lar units  composing  it,  it  becomes  impossible  for  such 
excreta  to  find  their  way  to  any  particular  spot,  or 
even  to  diffuse  through  the  tissues  with  sufficient 
rapidity  to  prevent  the  ill  effects  mentioned  above 
("  intoxication  ").  Definite  channels  are  found  in 
most  Metazoa  through  which  these  products  pass. 
In  the  tapeworm  and  its  allies  these  tubes  are  very 
delicate  and  permeate  the  body  in  all  directions, 
opening  to  the  exterior  in  pores.  Each  tube  is 
thought  to  be  the  differentiation  of  a  single  cell  and 
each  terminates  (or  rather,  more  accurately,  origi- 
nates) in  a  so-called  "  flame-cell."  This  cell  has  a 
wisp  of  cilia  projecting  into  the  cavity  of  the  tube, 
by  the  flickering  movement  of  which,  suggestive  of  a 
candle  flame,  the  absorbed  fluids  are  urged  down  the 
tube  and  ultimately  to  the  exterior. 

In  the  great  group  of  the  Annelids,  of  which  the 
earthworm  is  the  most  familiar  example,  the  body  is 
composed  of  a  great  number  of  segments  arranged 
like  a  row  of  pill-boxes  strung  on  a  tube,  the  tube 
being    the    alimentary    canal.     Each    segment,  al- 


120  GENERAL  BIOLOGY 

though  a  component  part  of  a  single  organism,  has  a 
certain  structural  individuality  of  its  own,  and  is 
separated  from  its  neighbors  on  either  side  by  a 
partition  wall,  the  septum.  The  inclosed  space, 
the  "  body-cavity,"  is  filled  with  a  watery  fluid  and 
is  lined  with  a  network  of  blood-vessels  through 
whose  walls  the  circulating  waste-products  transfuse 
into  this  fluid.  In  each  segment  there  is  found  an 
open-mouthed,  trumpet-shaped  tube,  fringed  with 
strong  cilia,  which  passes  through  the  septum  into 
the  cavity  of  the  segment  just  behind,  and  after  a 
more  or  less  convoluted  course  opens  directly  through 
the  body- wall  of  that  segment  to  the  exterior. 
The  action  of  the  cilia  creates  a  current  in  the  fluid 
which  passes  down  the  tube,  and  drains  off  to  the 
exterior  the  liquid  contents  of  the  body-cavity,  laden 
with  wastes.  Such  an  excretory  apparatus  is  of 
very  widespread  occurrence  in  various  groups  of 
animals  and  is  called  a  nephridium  (plural,  nephridia) . 
Such  a  nephridium  is  found  only  in  the  most 
simply  organized  annelids.  In  the  higher  annelids 
we  find  the  mechanism  much  improved  by  the  elimi- 
nation of  unnecessary  steps  and  the  securing  of  greater 
economy  of  energy  and  increased  efficiency.  Thus 
in  the  earthworm,  branches  of  the  blood-vessels 
surround  portions  of  the  nephridial  tube,  and  the 
major  portion  of  the  excreted  substances  transfuse 
directly  through  the  walls  of  the  blood-vessels  into 
the  cavity  of  the  nephridium,  instead  of  transfusing 
first  into  the  body-cavity  The  ciliated  funnel  in 
such  a  case  may  become  almost  (though  not  entirely) 


TISSUE-DIFFERENTIATION  121 

functionless.  This  is  much  the  same  improvement 
that  would  be  effected  in  the  transport  of  grain  to 
ships  if,  instead  of  unloading  the  sacks  of  wheat  from 
the  cars  to  the  dock  and  then  again  to  the  ship,  an 
arrangement  were  made  whereby  the  grain  could  be 
poured  directly  from  the  cars  into  the  hold. 

In  the  vertebrates  a  comparable  relation  of  blood 
system  and  excretory  tubules  is  found.  Instead  of 
being  distributed  throughout  the  body,  these  tubules 
are  concentrated  into  a  single  organ,  the  kidney; 
the  blood-vessels  supplying  the  kidney  develop  tiny 
knots  of  capillaries  called  glomeruli  (Latin,  glomeru- 
lus, "  little  ball  "),  which  afford  a  very  large  surface 
in  a  small  space  and  thus  permit  the  diffusion  of  the 
maximum  of  waste  substances  in  a  minimum  of 
time.  In  the  higher  vertebrates  there  is  no  trace 
of  the  openings  of  these  tubules  into  the  body-cavity. 
In  lower  forms,  however,  such  as  the  frog,  they  may 
be  observed  on  the  surface  of  the  kidney  although 
they  are  functionless. 

II.    Differentiation  in  Plants 

Plants  differ  strikingly  from  animals  in  the 
emphasis  which  evolution  has  laid  upon  various 
functions  common  to  both.  Animals  are,  for  the 
most  part,  actively  moving  creatures,  seeking  food 
in  various  places,  and  consequently  endowed  with 
elaborate  systems  of  differentiated  protoplasm  in 
the  form  of  muscles  and  sense-organs.  In  plants,  on 
the  other  hand,  the  assimilative  ("vegetative") 
function  is  predominant,   and  the  manifold  differ- 


122  GENERAL  BIOLOGY 

entiations  which  we  find  are  nearly  all  in  the  line  of 
structures  for  elaborating,  taking  up,  and  storing 
foods,  and  of  supporting  the  plant-body  (omitting, 
of  course,  all  consideration  of  the  reproductive 
function,  which  will  be  discussed  in  a  later  chap- 
ter). 

Accordingly,  we  find  little  trace  of  nervous  system 
or  muscular  system.  Yet  some  plants,  like  the 
Mimosa  or  sensitive  plant,  react  to  stimuli  with  a 
definiteness  that  is  comparable  to  a  nervous  reflex 
in  animals.  The  leaf  of  the  "  Venus'  flytrap," 
when  an  unwary  insect  touches  it,  springs  shut  with 
such  suddenness  and  vigor  as  to  catch  the  prey  and 
hold  it  fast.  This  action  is  brought  about  by  three 
sensitive  spines  which  may  therefore  be  held  to  be 
analogous  to  animal  sense-organs.  A  number  of 
plants  have  the  habit  of  folding  their  leaves  at  night, 
in  "  sleep,"  the  stimulation  being  the  change  from 
daylight  to  darkness.  Moreover  stimuli  are  trans- 
mitted (usually  slowly,  to  be  sure)  from  one  por- 
tion of  the  plant  to  another.  Plant  tissues  may  be 
anesthetized,  and  when  stimulated,  they  show 
"  fatigue."  We  may  conclude,  therefore,  that  al- 
though the  basis  of  the  mechanism  may  be  entirely 
different  from  that  of  animals,  yet,  in  so  far  as  the 
plant  tissue  is  differentiated  sufficiently  to  receive 
stimuli  from  without  and  transmit  them  to  other 
parts  of  the  plant-body,  there  to  bring  about  an 
appropriate  response,  it  may  be  asserted  that  plants 
possess  a  rudimentary  nervous  system.  They  differ 
from  most  animals  in  lacking  any  sort  of  a  coordinat- 


TISSUE-DIFFERENTIATION  123 

ing  or  central  nervous  system ;  that  is,  the  impulse  is 
conveyed  directly  from  a  more  or  less  diffusely 
sensitive  area  to  the  tissue  which  reacts. 

Plant  Movement.  —  We  ordinarily  think  of  plants 
as  rooted  fast  in  the  ground.  Nevertheless,  apart 
from  the  lower  forms,  mostly  unicellular,  which 
move  as  do  many  Protozoa  by  means  of  the  lashing 
of  flagella,  a  little  observation  will  show  that  most 
plants  execute  movements  differing  in  intensity  from 
the  sharp  folding  of  the  leaflets  of  the  sensitive  plant 
to  the  gradual  circling  of  the  sunflower  head  as  it 
follows  the  sun  from  east  to  west  during  the  day,  or 
the  twisting  of  the  climbing  tendril.  No  such  special- 
ized tissue  as  muscle  is  to  be  found  in  plants,  but  all 
such  movements  are  to  be  attributed  to  changes  in 
the  water  content  of  masses  of  spongy  tissue,  such 
that  when  the  cells  are  full  of  water,  the  resistance 
of  the  walls,  or  turgor  of  the  tissue,  makes  it  stiff  and 
erect,  whereas  the  withdrawal  of  the  water  from  the 
tissues  causes  the  leaves  or  other  parts  to  become 
flaccid  or  droop.  A  difference  in  the  amount  of 
turgor  on  opposite  sides  of  the  stem  will  thus  cause 
the  stem  to  bend  to  one  side  or  the  other  according 
to  which  side  is  under  the  greater  tension.  Sudden 
movements  are  brought  about  by  rapid  alterations 
in  the  turgor  in  localized  areas  under  the  influence 
of  external  stimuli. 

Supporting  Structures.  —  In  animals,  as  we  have 
seen,  the  skeleton  consists  of  a  framework,  either 


124  GENERAL  BIOLOGY 

external  or  internal,  to  which  the  softer  tissues  are 
attached  or  by  which  they  are  inclosed.  This 
skeleton,  whether  of  chitin,  or  bone,  or  merely  of 
connective  tissue,  owes  its  rigidity  to  the  deposit 
among  the  living  cells  of  a  non-living  intercellular 
substance ;  the  cells  themselves  have  very  thin 
walls.  In  plants,  on  the  other  hand,  we  find  that 
the  intercellular  substance  is  laid  down  in  the  form 
of  dense  cell-walls  of  the  cells  themselves,  and  that 
nearly  the  whole  tissue  of  the  stem  or  root  is  in  one 
sense  skeleton.  This  intercellular  substance  is  a 
complex  carbohydrate  called  cellulose,  or  a  deriva- 
tive of  cellulose,  lignin,  and  although  it  forms  the 
cell-walls  it  is,  of  course,  not  living  substance  itself, 
any  more  than  the  plates  of  lime  in  bone.  Not  all 
the  cells  are  uniformly  developed  in  this  manner. 
In  most  plant  tissues  there  has  arisen  a  differentiation 
of  the  supporting  or  "  mechanical  "  tissue  which 
frequently  occurs  in  bundles  or  strands  of  fibers, 
constituting  a  sort  of  internal  skeleton.  Hemp  and 
flax  are  abundantly  supplied  with  these  fibers,  which 
provide  us  with  linen,  hempen  cord,  etc.  On  the  out- 
side of  roots  and  stems,  particularly  of  the  larger 
plants  (trees),  the  cell- walls  become  enormously 
thickened,  with  an  accompanying  diminution  of  the 
protoplasmic  substance,  to  form  bark  or  cork. 
The  cork  is  impervious  to  water  and  may  be  com- 
pared with  a  secreted  exoskeleton,  like  chitin,  which 
protects  the  softer  living  portion  beneath.  In  trees 
the  whole  central  part  of  the  stem  is  composed  of 
solid  supporting  tissue  (wood),  the  living  portion  of 


TISSUE-DIFFERENTIATION 


125 


Fig.  45.  —  Stereogram  of  the  leaf  of  an  Iris:  e,  epidermis  ;  c,  cuticle  ; 
p,  palisade  cells  filled  with  chlorophyll-bodies ;  col,  collecting  cells ; 
conv,  conveying  cells  which  assist  in  the  transfer  of  the  synthesized 
sugars  from  their  place  of  origin  to  the  veins  ;  sh,  conducting  sheath  of 
vein  ;  h,  wood  of  vein  ;  b,  bast  or  woody  fibrous  portion  of  vein  ;  o,  air- 
space ;  s,  stoma  ;  g,  guard  cells.  —  (Osterhout.) 


126  GENERAL   BIOLOGY 

the  trunk  being  confined  to  the  comparatively  narrow 
layer  between  this  and  the  bark. 

In  the  leaves  the  skeleton  assumes  the  form  of 
branching  "  ribs  "  or  "  veins."  In  some  groups, 
e.g.  the  grasses,  these  are  single  and  unbranched ; 
in  others  they  form  a  network. 

Circulatory  System.  —  In  plants,  as  in  animals,  in 
those  species  which  are  of  such  a  size  that  substances 
do  not  diffuse  readily  through  their  tissues,  there  is  a 
system  of  tubes  which  permit  the  circulation  of 
liquids  from  one  place  to  another.  The  movement 
of  the  circulating  liquids  is  due  to  mechanical 
agencies  external  to  the  protoplasm.  There  is,  of 
course,  nothing  corresponding  to  a  heart.  The  tubes, 
which  are  of  various  sorts  and  positions,  often  occur 
in  bundles.  They  are  formed  by  the  coalescence  of 
cells,  end  to  end,  and  the  subsequent  dying  out  of  the 
living  contents.  The  prime  factor  in  causing  the 
ascent  of  water  in  the  plant  tissue  is  the  "  transpira- 
tion "  or  evaporation  of  water  from  the  leaves. 
Just  how  this  operates  is  still  a  complicated  and  un- 
solved problem.  A  result  of  such  movements  of 
liquid  is  the  transportation  of  food  substances  to 
various  portions  of  the  plant-body. 

Alimentary  System.  —  Algse  derive  their  food  from 
the  surrounding  medium,  and  their  tissues  are  cor- 
respondingly soft  and  permeable.  There  is  for  this 
reason  no  localized  area  for  the  alimentary  processes. 
In  land  plants,  on  the  other  hand,  there  is  a  twofold 


TISSUE-DIFFERENTIATION  127 

source  of  food,  the  soil  and  the  air.  The  plant  can 
take  in  food  from  the  soil  only  in  solution,  and  such 
dissolved  substances  can  transfuse  only  through  very 
thin  cell- walls.  Accordingly  the  roots  of  such  plants 
are  covered  with  delicate  threadlike  processes  called 
root-hairs,  each  of  which  is  composed  of  a  single 
thin-walled  cell  and  is,  indeed,  merely  an  extension  of 
a  cell  of  the  skin  of  the  rootlet  itself.  These  root- 
hairs  are  quite  short-lived  and  are  to  be  found 
therefore  only  in  the  youngest  root-branches.  The 
organs  for  taking  in  food  from  the  air  are  the  green 
leaves.  The  upper  and  lower  surfaces  of  the  leaf 
are  usually  made  up  of  rather  stiff  cells,  forming  a 
cuticle,  in  which  are  found  numerous  mouthlike 
openings  between  the  cells,  called  stomata,  that  lead 
to  air-spaces  within  the  leaf.  The  body  of  the  leaf  is 
made  up  of  numerous  irregular  or  elongate  cells, 
loosely  packed  together,  and  crowded  with  green 
chloroplasts  or  chlorophyll  bodies,  the  means  whereby 
the  carbon  dioxide  of  the  air  is  fixed  and  converted  into 
carbohydrate  (see  Chapter  11). 

The  substances  taken  in  through  the  roots,  and  the 
sugars  and  starches  formed  in  the  leaves,  are  dis- 
tributed throughout  the  plant-body  by  means  of  the 
circulatory  system  mentioned  above.  In  lower 
plants  with  relatively  undifferentiated  tissues  such 
a  distribution  of  substances  must  take  place  by  direct 
transfusion  through  the  cell- walls  themselves.  As  in 
animals,  however,  food  is  not  taken  in  continuously 
in  higher  plants,  at  least  not  in  the  leaves.  The 
formation  of  sugar  depends  upon  sunlight,  and  ceases. 


128  GENERAL  BIOLOGY 

of  course,  at  night.  During  the  day  the  sugar  is 
usually  converted  into  starch  and  stored  up  in  the 
leaves  as  such.  During  the  night,  however,  this 
excess  is  converted  again  into  sugar  and  carried 
away  to  nourish  the  plant  elsewhere.  As  in  animals, 
various  parts  of  the  plant  may  serve  as  storehouses. 
Accumulations  of  food  are  usually  found  in  seeds, 
where  their  presence  is  of  manifest  advantage  to  the 
developing  plant.  Under  the  influence  of  certain 
fungi  the  underground  stems  of  certain  plants  {e.g. 
the  potato)  thicken  up  and  accumulate  starch. 
Such  accumulations  of  "  reserve  "  food  may  not 
always  be  starch.  They  may  be  sugar  (sugar  beet) 
or  fat  (cotton  seed). 

Some  plants,  including  the  whole  group  of  fungi 
(molds,  etc.),  draw  their  food  supply  from  other 
plants  or  animals.  As  they  get  their  food  at  second- 
hand, already  elaborated,  they  lack  or  have  lost  the 
special  structures  by  means  of  which  other  plants 
manufacture  their  own  food. 


CHAPTER  VI 

ONTOGENESIS 

There  is  no  reason,  a  priori,  why  the  individual 
plant  or  animal,  barring  accident,  should  not  live 
forever.  We  can  conceive  of  a  perfectly  balanced 
metabolism  in  which  the  up-building  or  tissue- 
repairing  process  would  exactly  balance  the  dis- 
ruptive or  tearing-down  process.  As  a  matter  of 
fact  we  know  of  no  form,  even  among  the  simplest, 
of  which  this  is  true.  In  all,  katabolism  after  a 
certain  time  tends  to  exceed  anabolism.  In  spite  of 
Nature's  wonderful  recuperative  powers,  the  living 
organism,  like  a  machine,  tends  to  wear  out,  and  after 
a  brief  period  is  fit  only  for  the  scrap-heap. 

Competition,  the  "  struggle  for  existence,"  is 
severe  among  different  species,  and  a  species  that  is 
able  to  replace  frequently  its  worn-out  members  with 
vigorous  new  individuals  would  maintain  its  level 
of  efficiency,  so  to  speak,  at  the  highest  point.  More- 
over, where  numbers  count  so  heavily,  the  species 
that  might  be  able  to  replace  each  worn-out  veteran 
with  a  hundred  or  a  thousand  new  recruits,  would 
have  a  corresponding  advantage  over  one  that  could 
not  do  so. 

Whether  or  not  some  such  conditions  as  these  may 
have  been  the  cause,  they  indicate,  at  any  rate,  an 

K  129 


130  GENERAL  BIOLOGY 

advantage  to  the  species  of  the  phenomenon  of 
reproduction,  —  the  replacement  of  individuals  by 
others  from  the  same  stock.  Each  individual  passes 
through  a  cycle  of  birth,  youth,  maturity,  senility, 
and  dissolution,  but  before  the  final  stages  are 
reached,  it  normally  produces  other  individuals  to 
take  its  place. 

Biogenesis.  —  This  phenomenon  clearly  implies  a 
continual  stream  of  life,  in  which  individual  succeeds 
individual  like  waves  on  the  ocean.  The  physical 
connection  of  the  individual  with  its  ancestry  is  thus 
obvious.  An  aphorism  of  a  century  ago  expresses 
it,  —  "  Omne  vivum  ex  vivo,''  "  all  life  from  [pre- 
existing] life."  But  a  contrary  view  was  also  held 
until  very  recent  times,  viz.  that  living  organisms 
may  arise  from  non-living  matter.  This  conception, 
which  has  been  called  spontaneous  generation  or 
abiogenesis,  arose  from  incomplete  or  misinterpreted 
observation.  Thus  it  is  a  matter  of  everyday  ob- 
servation that  maggots  develop  in  rotting  meat. 
Whence  do  they  come,  if  not  from  the  meat  itself? 
A  generation  less  well  trained  in  the  methods  of 
exact  research  found  no  difficulty  in  accepting  just 
such  an  hypothesis,  but  an  ingenious  Italian,  Redi, 
showed  that  if  the  meat  be  covered  with  gauze  so  as 
to  keep  out  the  blow-flies,  no  maggots  ever  develop, 
since  these  are  produced,  not  from  the  meat,  but 
only  from  the  eggs  which  the  fly  lays  on  the  meat. 
In  brief,  it  has  been  conclusively  shown,  in  every 
instance,  that  no  living  forms  are  to  be  found  to-day. 


ONTOGENESIS  131 

except  such  as  have  arisen  from  preexisting  individ- 
uals of  the  same  species.^ 

Individuals  give  rise  to  other  individuals  either  by 
simply  cutting  in  two  to  form  two  half-organisms, 
each  of  which  reorganizes  its  tissue  so  as  to  complete 
itself  to  the  specific  type,  or  by  budding  off  a  small 
portion  of  itself,  which  grows  and  differentiates  to  a 
form  similar  to  its  parent,  or,  finally,  by  budding  off 
a  single  cell,  which  grows  and  differentiates  into  an 
individual  like  its  parent. 

Reproduction  as  a  Growth  Process.  —  It  was 
pointed  out  in  a  previous  chapter  that,  in  most  cases, 
cell  division  is  a  consequence  of 
cell  growth  or  "  cvmiulative  in- 
tegration "  and,  in  fact,  may  be 
considered  as  a  phase  of  the  growth 
phenomenon,  discontinuous  instead 
of  continuous.  The  result  is  to 
increase  the  mass  of  a  tissue  with-  ^^J^H  uhl^rltTnl 
out  differentiation,  and  without  simple  budding.— 
the  alteration  of  the  size-relations 
of  its  components,  the  cells.  In  the  case  of  free- 
living  one-celled  organisms,  cell  division  results  from 
the  same  cause,  —  with  the  difference  that  the  newly 
formed  cell  units  are  free  individuals  instead  of 
elements  of  a  mass.  This  is,  however,  a  distinction 
without  any  real  difference.  On  the  other  hand, 
certain  unicellular  organisms,   after  fission,   remain 

^  Or  in  rare  cases  as  mutants  from  closely  related  species.  See  next 
chapter. 


132 


GENERAL  BIOLOGY 


attached  to  one  another  in  chains  or  masses.  Such 
a  mass  might  justly  be  called  a  tissue,  except  that  it 
is  convenient  to  restrict  the  use  of  that  term  to  a 
cell-mass  which,  in  turn,  is  a  part  of  a  still  more 
highly  organized  complex.  In  certain  Algae  this 
connection  of  cells  with  one  another  is  transitory 
and  indefinite.  In  some  forms,  however,  the  con- 
nection is  permanent, 
and  the  mass  con- 
sists of  a  definite 
number  of  cells.  In 
such  a  case  we  speak 
of  the  cell  group  as 
a  colony.  The  com- 
bination of  individ- 
uals in  a  colony  is 
not  only  found  in  the 
Protista,  but  in  many 
of  the  higher  groups 
as  well,  particularly 
in  such  forms  as  are 
fixed  to  one  spot  (sea  squirts,  coelenterates,  sponges, 
and  most  plants). 


Fig.  47.  —  Budding  in  an  animal 
(Clavelina).  The  individual  organisms 
develop  from  an  outgrowth  of  the  body- 
wall  analogous  to  a  plant  rootstalk. 
Natural  size. —  (Herdman.) 


Fission  in  Metazoa.  —  Not  only  in  the  Protista, 
but  in  metazoa  also,  are  found  examples  of  repro- 
duction by  fission.  In  Ctenodrilus,  one  of  the  lower 
Annelids,  the  worm  cuts  in  two  by  transverse  fission 
much  as  an  elongated  protozoan.  The  separate  por- 
tions of  the  original  body  then  become  transformed 
into  complete  individuals  by  a  shifting   and   read- 


ONTOGENESIS 


133 


justment  of  the  original  tissues,  some  of  which 
become  modified  to  wholly  different  uses  to  what 
they  served  before. 


Fig.  48.  —  Vegetative  reproduction  (terminal  budding)  in  an  Annelid 
worm,  Myrianida:  A,  an  asexual  individual  which  has  produced  by 
budding  from  a  zone  (2)  a  chain  of  twenty-nine  zooids,  the  oldest  being 
labelled  1 ,  the  youngest  29 ;  B,  a  ripe  male  zooid  ;  C,  a  ripe  female 
zooid.  —  (Malaquin.) 


In.  Myrianida,  another  worm  of  the  same  class, 
the  divisions  follow  each  other  so  rapidly  that  one 


134  GENERAL  BIOLOGY 

individual  is  not  budded  off  before  another  constric- 
tion becomes  visible,  and  the  result  is  a  chain  of 
individuals  in  various  stages  of  differentiation. 

It  is  evident  that  if,  instead  of  separating  from  the 
parent  organism,  the  newly  formed  "  sub-individ- 
uals "  remain  attached,  we  would  have  a  resultant 
organism  of  a  different  character,  compounded,  so 
to  speak,  of  individual  units  held  together  by  a 
sort  of  common  bond.  It  is  supposed  by  some 
that  the  segmental  structure  more  or  less  evident  in 
nearly  all  the  animal  kingdom  came  about  originally 
through  some  such  suppressed  fission.  Just  as  we 
have  seen  that  whereas  the  stress  of  growth  and  the 
necessity  for  maintaining  a  certain  relation  between 
nucleus  and  cytoplasm  results  usually  in  the  cleav- 
age of  the  cell,  but  on  the  other  hand  results  some- 
times in  the  suppression  of  this  cleavage  through  a 
distribution  of  the  nuclear  substance  to  form  multi- 
nucleate cells  or  syncytia,  so  also  conditions  of 
existence  that  in  the  beginning  brought  about  a 
cleavage  of  the  whole  organism  (schizogeny)  also 
may  have  made  it  advantageous  for  the  separate 
parts  to  remain  together.  We  can  trace  different 
degrees  in  such  a  condition,  from  Ctenodrilus,  in 
which  the  separation  is  immediate,  or  Myrianida, 
in  which  it  is  temporarily  retarded,  to  the  tape- 
worm, in  which  the  segments  are  permanently 
attached  together  to  form  a  "  segmented  body." 
In  the  tape-worm,  however,  the  segments  at  the 
posterior  end  may  drop  off  without  losing  such  vital 
functions  as  they  are  endowed  with,  and  the  number 


ONTOGENESIS 


135 


of  segments  in  the  body  apparently  has  no  influence 
on  or  relation  to  the  individuality  of  the  organism. 
In  the  higher  animals  this  primitive  segmentation 
has  become  overshadowed  by  the  unity  of  the  whole, 
so  that  no  segment  could  be  sacrificed  without 
destroying  the  individual.  This  type  of  bodily 
structure  is  called  metameric  segmentation,  and  is  the 
fundamental  plan  in  nearly  all  the  animal  phyla/ 


Fig.  49.  —  Diagram  of  metameric  segmentation  (as  of  the  earth- 
worm) :  A,  longitudinal  section  of  the  body  showing  the  alimentary 
canal  and  coelom  divided  by  septa ;  B,  cross-section  ;  coe,  ccelora  ;  al, 
alimentary  canal  ;  w,  mouth  ;  an,  anus.  —  (From  Sedgwick  and  Wilson.) 


from  the  generalized  Annelids,  in  which  the  segments 
bear  to  the  body  somewhat  the  same  relation  that 
the  individual  cars  do  to  a  vestibuled  train,  to 
the  higher  vertebrates,  in  which  the  segmental 
feature  is  plainly  evident  only  in  the  developmental 
stages. 


Fission  in  Lower  Plants.  —  In  the  important 
group  of  the  Bacteria,  multiplication  takes  place 
exclusively  by  fission.  In  the  spherical  type  (Coc- 
cus)   the    planes    of    division    may    be    repeatedly 


^  The  exceptions  are  the  higher  Mollusca,  Flatworms,  Coelenterates, 
and  Sponges. 


136  GENERAL   BIOLOGY 

transverse,  producing  strings  or  chains  of  cells 
{Streptococci),  or  they  may  be  alternatingly  in  two 
vertical  planes,  producing  groups  of  four,  or,  finally, 
the  planes  of  division  may  occur  in  the  three  dimen- 
sions of  space,  producing  groups  of  eight  (Sarcina) 
in  a  cubical  aggregate. 


Fig.  50.  —  Types  of  Bacteria,  illustrating  various  modes  of  fission  : 
A,  simple  coccus  form;  B,  diplococcus  (fission  in  one  plane  only,  the  pair 
separating  as  formed;  E,  chain  form  (streptococcus),  the  fission  all  in  one 
plane,  but  the  cells  remaining  together;  C,  division  in  fours  (tetrads)  in 
one  plane  ;  D,  division  in  three  planes  to  produce  cubes  or  bale-like 
masses  (sarcina)  ;  G,  division  planes  at  random,  to  produce  masses 
(staphylococcus) ;  F,  flagellated  cocci  (nitrogen-fixing  bacteria  of  the  soil) ; 
H,  rod-form  (bacillus) ;  /,  bacillus  with  flagella  at  one  end  (putrefactive 
bacteria);  /,  with  flagella  all  over  the  cell  body  (typhoid  bacillus). 


Budding.  —  From  reproduction  by  fission  of  the 
parent  body,  to  reproduction  by  cutting  off  of  a 
small  portion  of  the  body,  is  a  short  step.  In  the 
latter  case  the  individual  identity  of  the  parent 
organism  is  unaffected,  and  the  part  budded  off 
develops  into  the  specific  type  by  individual  growth 
and  differentiation.  Such  a  process  of  reproduction 
is  known  as  gemmation  or  budding  and  is  well-nigh 
universal  in  the  plant   kingdom  as  well  as  in  those 


ONTOGENESIS 


137 


animal  types  that  have  a  plantlike  habit,  such  as 
the  sponges,  coelenterates,  and  tunicates.  Familiar 
examples  are  the  "  runners  "  of  strawberries,  the 
"  shoots  "   of  willows   and  other  trees,   the  tubers 


Fig.  51.  —  Habit  of  growth  of  a  grass:  A,  aerial  stem  terminating  in 
a  branched  inflorescence;  B,  underground  stem  or  rhizome  sending  up  a 
new  shoot  from  one  of  the  nodes;  C,  section  through  the  node  of  a  stem 
that  has  been  placed  horizontal,  showing  the  sheathing  leaf  base,  I,  and 
the  beginning  of  the  upward  curvature  of  the  stem.  —  (From  Curtis.) 


of  potatoes,  etc.  The  most  primitive  condition  in 
plants  was  doubtless  that  of  a  plant-body  (thallus), 
dying  or  drying  up  in  the  middle,  and  leaving  two 


138 


GENERAL  BIOLOGY 


or  more  vigorous  extremities  to  pursue  an  individual 
existence.  The  development  of  runners  and  shoots 
is  a  special  modification  of  the  same  process. 

In  the  animal  realm,  one  of  the  simplest  examples 
of  budding  is  found  in  the  common  fresh-water 
Hydra.     In   this   form,    the   interstitial    cells    at   a 


Fig.  52.  —  Budding  Hydra,  as  seen  under  the  low  power  of  a  com- 
pound microscope:  B,  attached  end;  B^,  B^,  buds;  M,  mouth;  T,  tenta- 
cles.—  (From  Hunter's  Elements  of  Biology :  American  Book  Co.) 


certain  point  begin  to  increase  in  numbers,  and  form 
a  knoblike  protrusion  on  the  side  of  the  hydra-body. 
By  a  shifting  and  rearrangement  of  the  cells  composing 
this  lump,  a  cavity  forms,  in  direct  communication 
with  the  inner  cavity  of  the  Hydra.  This  bud 
grows  until  it  attains  a  considerable  size,  when  the 
cells  at  the  extremity  begin  to  differentiate  into  ten- 


ONTOGENESIS  139 

tacles,  a  mouth  opens,  and  the  bud  becomes  in  all 
respects  an  individual  Hydra,  able  to  carry  on  all  the 
functions  of  life.  The  process  is  completed  by  a 
severing  of  the  connection  between  bud  and  parent. 

Permanent  Budding.  —  In  the  majority  of  the 
marine  relatives  of  the  Hydra  the  buds  formed  in 
this  way  do  not  separate  from  the  parent  stem,  but 


Fig.  53.  —  Portion  of  Syllis  ramosa,  an  annelid  worm  in  which  the  col- 
lateral buds  branch  repeatedly.  —  (M'Intosh.) 


remain  attached,  forming  colonies  of  many  indi- 
viduals. Such  a  condition  of  persistent  budding  is 
characteristic  of  plants,  but  occurs  as  well  in  many 
Protozoa.  In  some  instances  the  buds  run  together 
in  such  a  fashion  that  it  is  impossible  to  discriminate 
one  individual  from  another.  In  Syllis  we  have  a 
highly  organized  worm  that  divides  and  subdivides, 
like  'the  branches  of  a  tree,  while  individual  heads 
develop  here  and  there. 


140  GENERAL  BIOLOGY 

Spore-formation.  —  In  many  types  of  Protista 
a  modification  of  reproduction  by  fission  is  found, 
which  is  especially  advantageous  in  certain  condi- 
tions of  existence.  After  repeated  divisions  of  in- 
dividuals in  a  free  state,  the  organism  may  surround 
itself  with  a  thick  wall,  forming  a  cyst  within  which 
the  protoplasm  fragments  into  a  multitude  of  minute 
particles  called  spores^  each  of  which  has  the  poten- 
tiality of  developing  into  an  individual  like  that 
which  formed  the  cyst.  In  this  way  a  species  may 
tide  over  a  critical  period,  as  of  drouth,  in  such 
an  encysted  condition.  When  favorable  conditions 
again  intervene  (perhaps  after  several  years),  the 
multitude  of  emerging  spores  insure  the  immediate 
existence  of  a  large  number  of  individuals,  and  thus 
reduce  the  chances  of  extermination  for  the  race. 
In  the  bacteria,  under  certain  conditions,  the  proto- 
plasm of  the  tiny  cell  condenses  into  one  or  more 
spores,  which,  on  account  of  their  minute  size,  may  be 
blown  about  in  the  dust,  and  thus  afford  the  most 
effective  means  for  the  dispersal  of  the  species. 
Propagation  by  spores  is,  indeed,  a  feature  of  devel- 
opment throughout  the  plant  kingdom. 

Plants  are  adapted  in  large  measure  to  what  we 
call  a  "vegetative"  existence;  that  is,  with  the 
exception  of  the  Protophyta,  most  of  which  are 
motile,  they  are  fixed  in  the  place  where  they  sprout 
and  cannot  seek  food  elsewhere  if  it  fails,  or  avoid 
extremes  of  climate  by  migrating.  This  disad- 
vantage is  compensated  by  the  production  of  spores, 
which  are  frequently  developed  in  enormous  numbers, 


ONTOGENESIS  141 

and,  being  scattered  by  various  agencies  far  and  wide, 
bring  about  an  extensive  distribution  of  the  species. 
The  method  of  propagation  just  outhned,  whether 
by  spores  or  by  buds,  by  simple  or  multiple  fission, 
is  termed  vegetative  or  asexual  reproduction.  We 
may  define  it  as  the  cutting  off  from  an  organism  of 
a  single  mass  of  protoplasm  (of  one  or  many  cells) 
which  independently  differentiates  into  an  organism 
resembling  its  parent.  The  greatest  diversity  in 
vegetative  reproduction  is  to  be  observed  in  the 
various  groups  of  animals  and  plants,  but  in  every 
case  it  can  be  explained  as  a  special  form  of  the 
growth  process. 

Sexual  Reproduction 

In  nearly  all  forms  of  organic  life,  reproduction  is. 
complicated  by  an  accompanying  phenomenon,  the 
meaning  of  which  is  not  at  all  clear.  In  the  bacteria 
and  a  few  other  Protista  vegetative  reproduction 
is  the  only  sort  known.  In  all  others  we  find  that, 
either  accompanying  the  vegetative  reproduction, 
or  alternating  with  it,  there  occurs  the  production  of 
certain  reproductive  cells  (germ-cells)  to  which  the 
name  gamete  is  given.  Two  of  these  gametes  fuse 
together,  either  completely  or  partially,  and  from  the 
fused  cell  (called  the  zygote)  a  new  individual  devel- 
ops, or,  more  accurately,  the  zygote  transforms  by 
growth  and  differentiation  into  a  new  individual. 
When  the  two  cells  fuse  completely  into  one,  in  the 
formation  of  the  zygote,  the  process  is  termed  ho- 
log  amy.     When  the.  fusion  involves  only  the  nucleus, 


142  GENERAL  BIOLOGY 

it  is  called  Icaryogamy.  It  is  doubtful,  however,  if 
pure  karyogamy  ever  takes  place.  The  total  fusion 
of  gametes,  in  turn,  may  be  of  various  degrees  of 
specialization,  either  between  similar  cells  (isogamy) 
or  between  cells  of  different  ^  sizes  (anisogamy), 
leading  to  the  differentiation  of  sexually  specialized 
cells  (oogamy). 


Fig.  54.  —  Bodo  lens:  1 ,  normal  "  monad  ";  2,  same  in  the  semi-amoeboid 
condition  preparatory  to  conjugation ;  3,  two  individuals  in  the  process 
of  coalescence;  4,  the  resultant  zygote;  5,  a  zygote  whose  protoplasmic 
contents  have  divided  into  spores;  6,  young  monads  escaping  from  the 
sporocyst.  — ;  (After  Kent.) 

Total  Conjugation.  Isogamy.  —  Even  in  the 
"  lowest  "  of  the  Protista,  the  generalized  condition 
in  which  two  similar  adults  conjugate  to  produce 
a  zygote  is  very  rare.  In  such  a  case,  the  entire 
organism  functions  as  a  gamete.  One  of  the  most 
primitive  examples  is  Bodo  lens.     In  this  species,  two 


ONTOGENESIS  14S 

free-swimming  individuals  (see  fig.  54)  come  to  a 
temporary  resting  position  with  one  flagellum  touch- 
ing solid  matter.  They  then  sway  toward  each 
other,  meet,  and  fuse  into  one.  Their  flagella  dis- 
appear, and  a  thick  covering  or  cyst  is  secreted  about 
the  fused  mass.  After  a  period  of  rest,  the  contained 
protoplasm  fragments  into  a  number  of  spores,  or 
into  individuals  like  the  parents,  which  escape  from 
the  cyst  as  typical  "  monads  "  of  a  smaller  size. 
Other  Bodos  show  a  certain  difference  in  size  between 
individuals  of  the  same  culture.  Conjugation  occurs 
apparently  at  random  between  these  dissimilar  in- 
dividuals and  between  similar  ones. 

It  is  perhaps  more  usual  for  the  fully  developed 
microorganism,  instead  of  fusing  directly  with 
another  similar  individual,  to  fragment  into  a 
number  of  minute,  active,  individuals  called  micro- 
gametes,  which  conjugate  with  one  another.  The 
latter  procedure  must  be  of  advantage  to  the  species, 
since  on  account  of  the  much  larger  number  of  new 
individuals  produced  at  one  time,  the  race  is  not  so 
likely  to  be  exterminated. 

One  of  the  most  primitive  examples  is  Stephano- 
sphcera,  which  consists  of  a  colony  of  eight  flagellate 
individuals  (fig.  55)  arranged  in  a  plate  within  a 
gelatinous  sphere  of  which  they  form  the  equator. 
In  reproduction,  each  cell  of  the  colony  divides  into 
sixteen  or  thirty-two  smaller  individuals  (gametes), 
all  of  the  same  size,  which  break  out  of  the  gelati- 
nous'sphere,  swim  away,  and  conjugate,  two  by  two, 
to  form  a  zygote.      Each  zygote  divides  into  four 


144 


GENERAL  BIOLOGY 


free  individuals,  each  of  which  secretes  a  gelatinous 
sphere  and  by  successive  divisions  gives  rise  to  a  new 
eight-celled  colony. 

Anisogamy.  —  A  further  advantage  would  accrue 
to    the   species    that    developed    microgametes,    in 


Fig.  55.  —  Stephanosphcera,  a  colonial  flagellate  which  reproduces  by 
isogamy  :  A,  a  mature  8-celled  colony  in  equatorial  view;  B,  a  colony 
whose  individuals  by  repeated  divisions  have  formed  daughter-colonies 
within  the  mother  colony  ;  C,  a  colony,  whose  individuals  by  repeated 
divisions  have  formed  flagellated  individuals  that  function  as  gametes  ; 
D,  single  gamete  on  a  larger  scale  ;  G  to  L,  stages  in  the  process  of  con- 
jugation. —  (From  Hertwig.) 


comparison  with  one  that  did  not,  in  that  the  larger 
number  of  individuals  so  produced  would  be  more 
active  on  account  of  their  smaller  size,  and  therefore, 


ONTOGENESIS  145 

because  of  their  greater  number,  would  have  more 
chances  for  meeting  and  conjugating  with  one 
another,  assuming,  as  is  probably  the  case,  that  their 
meeting  is  purely  accidental.  But  this  sort  of  a 
specialization  would  also  have  its  drawback,  arising 
from  the  fact  that  the  reduction  in  size  of  the  micro- 
gamete  also  involves  a  reduction  in  the  amount 
of  reserve  food  material  stored  within  the  cell. 
The  latter  is  an  important  consideration  in  tiding 
the  zygote  over  the  critical  period  of  encystment, 
and  on  this  account  it  is  evident  that  in  such  a 
form  as  the  Bodo  just  described  the  zygote  arising 
from  the  accidental  fusion  of  a  larger  gamete 
(megagamete)  with  a  microgamete  would  stand 
a  better  chance  of  surviving  and  perpetuating  the 
race  than  the  zygote  formed  of  two  fused  micro- 
gametes.  As  a  matter  of  fact,  we  find  that  in  the 
great  majority  of  existing  forms  zygosis  ^  occurs  be- 
tween dissimilar  gametes,  —  a  condition  we  have 
called  anisogamy.  The  distinction  is  very  likely 
more  than  that  of  mere  size,  and  probably  involves 
subtle  differences  in  protoplasmic  organization,  but 
our  conclusions  in  that  regard  are  wholly  inferential. 
Even  in  isogamy,  the  two  conjugating  gametes 
may  be  physiologically  different;  that  is,  the  mor- 
phological difference  may  develop  secondarily. 

1  The  partial  or  complete  fusion  of  two  gametes  is  more  often  referred 
to  as  "fertilization,"  but  this  word,  which  represents  an  archaic  con- 
ception of  development,  is  misleading.  Zygosis,  a  word  coined  by 
Lankester  to  denote  the  fusion  of  the  gametes  to  form  the  zygote,  is 
preferable  because  it  describes  the  phenomenon  without  interpreting  it. 

L 


146  GENERAL  BIOLOGY 


f 

i 

'■ 

f 

1  \ 

Fig.  56.  —  Conjugation  of  two  strands  of  Spirogyra.  The  upper  cells 
are  just  forming  the  conjugating  tube.  This  has  been  established  in  the 
next  couple  of  cells.  In  the  next  two  pairs  of  cells  the  cell-protoplasm  is 
passing  over  from  one  strand  to  the  other.  In  the  lowermost  cell  this 
process  has  been  completed,  and  the  cell-contents  have  fused  into  a 
zygote,  which  secretes  a  thick  wall  about  itself  and  becomes  a  resting 
spore. 

Fig.  57.  —  Conjugation  of  Mougeotia.  In  this  form  the  cell-contents 
fuse  midway  between  the  two  conjugating  strands. 

The  difference  between  the  gametes  may  be  of 
varying  degrees.     In  Spirogyra,  in  which  the  long 


ONTOGENESIS 


147 


filaments  approximate  one  another,  and  the  contents 
of  the  cell  of  the  one  strand  flow  out  into  those  of 
the  other,  forming  a  chain  of  zygotes  (fig.  56),  the 
condition  is  almost  that  of  isogamy,  the  only  differ- 
ence between  the  gametes  being  that  of  relative 
passivity  and  activity.  If  the  protoplasm  of  both 
strands  flowed  out  and  fused  midway,  we  would  have 
an  example  of  pure  isogamy.  Such  a  condition  is 
foimd  in  some  species  of  a  relative  of  Spirogyra, 
Mougeotia. 


Fig.  58.  —  Eudorina:  A,  a  mature  colony  (from  nature);  B,  formation 
of  the  two  kinds  of  reproductive  cells. 


Sexual  Differentiation.  —  In  Eudorina  (fig.  58) 
we  have  a  colony  of  sixteen  to  sixty-four  flagellate 
cells,  that,  like  Stephariosphoera,  are  imbedded  in  a 
gelatinous  sphere  which  they  secrete.  There  are, 
however,    two    kinds    of    these    colonies    from    the 


148  GENERAL  BIOLOGY 

standpoint  of  reproduction.  In  one  kind,  the  cells 
dmde-  into.  mirrngRmetps^  in  thp  ntlipr^  mpgaga- 
metes  ^  result  fronx-the^^taking  up  of  reserve  food  by 
the  individual  cells  of  the  colony.  When  two  such 
differentiated  colonies,  in  their  aimless  wanderings, 
bump  into  each  other,  they  stick  together,  and  the 
microgametes  of  one  colony  separate,  penetrate 
the  gelatinous  envelope  of  the  other,  and  fuse  with 
the  megagametes  there.  Each  resultant  zygote, 
after  a  resting  period,  divides  into  a  sixteen-  or 
thirty-two-cell  colony. 

In  Volvox,  a  closely  allied  type,  the  differentiation 
of  the  two  kinds  of  gametes  has  reached  a  much 
greater  degree.  The  Volvox  colony  is  composed  of 
a  great  number  of  individual  units  (as  many  as 
22,000),  which  are  not  only  united  by  the  gelatinous 
matrix  in  which  they  are  immeshed,  but  by  proto- 
plasmic intercellular  connections  as  well.  In  au- 
tumn, a  score  or  more  of  these  cells  begin  to  grow 
by  the  accumulation  of  food  reserves,  until  they 
exceed  the  size  of  an  ordinary  cell  many  times 
(see  &g.  59).  Other  cells  reverse  the  process  and 
divide  repeatedly,  until  a  great  number  of  minute 
microgametes  result.  The  differentiation  is  not 
alone  one  of  size.  In  proportion  as  the  megagametes 
increase  in  bulk,  they  become  immobile  and  lose 
the  two  flagella  with  which  the  other  cells  are  pro- 
vided ;  on  the  other  hand,  the  cell-body  of  the  micro- 
gametes is  markedly  attenuated,  and  their  activity 
is     correspondingly    increased.     The    megagametes 

1  /i^yas  =  large ;  fiUpos  =  small. 


ONTOGENESIS 


149 


do  not  begin  to  develop  until  the  microgametes  have 
matured  and  broken  away  from  the  colony,  so  that 
little  or  no  opportunity  is  afforded  for  zygosis  to 


Fig.  59.  —  Volvox  globator,  a  large  colonial  flagellate:  A,  a  sexually 
ripe  colony,  showing  noicrogametes  (d")  and  macrogametes  (?),  in  vari- 
ous stages  of  development;  B,  a  portion  of  the  edge  of  the  colony  highly 
magnified,  showing  three  flagellate  cells  united  by  protoplasmic  threads, 
and  a  single  reproductive  cell,  rp;  st,  stigma  or  "eye-spot";  cv,  con- 
tractile vacuole. —  (From  Bourne,  after  Kolliker.) 

occur:  between  members  of  the  same  colony.  The 
fusion  of  the  passive  megagamete  and  the  active 
microgamete    occurs    within    the    colony    of    which 


150  GENERAL  BIOLOGY 

the  former  is  a  member,  and  the  resulting  zygote, 
becoming  encysted,  either  develops  directly  into  a 
"  daughter-colony,"  or  winters  over  in  a  cyst,  to 
produce  a  new  colony  the  next  spring.  In  such  a 
case,  the  multitude  of  the  swimming  cells  composing 
the  original  colony  perish.  We  see  here  a  funda- 
mental difference  between  a  colony  such  as  Eudorina, 
in  which  each  member  is  capable  of  functioning  as 
a  gamete,  and  of  reproducing  the  race,  and  Vol- 
vox,  in  which  only  a  very  few  of  the  cells  so  function, 
and  the  great  majority  perish  with  each  generation. 
This  condition  is  an  accompaniment  to  a  form  of 
specialization,  in  which  certain  cells  of  the  organism 
become  peculiarly  adapted  for  the  function  of  repro- 
duction. When  the  specialization  has  gone  on  so  far 
as  is  the  case  in  Volvox,  we  usually  speak  of  the  large 
passive  megagamete  as  an  egg,  and  the  tiny  active 
microgamete  as  a  sperm. 

Those  cells  of  the  aggregate  that  exercise  the 
reproductive  function  we  speak  of  as  germ-cells,  in 
contrast  to  the  somatic  cells,  which  have  lost  this 
function.  The  differentiation  has  thus  affected  the 
character  of  the  protoplasm  in  the  two  kinds  of 
cells,  one  kind  —  the  germ-plasm,  in  which  the  func- 
tion of  reproduction  is  especially  developed,  —  is, 
from  one  point  of  view,  immortal,  whereas  the 
soma-plasm  dies  with  each  generation  and  is  renewed 
with  each  generation  by  growth  from  the  germ- 
plasm. 


ONTOGENESIS 


151 


Partial  Conjugation 

Cytoplasmic  Conjugation  (Plastogamy) .  —  In  cer- 
tain cases  (slime-molds,  Heliozoa,  Rhizopoda)  two 
or  more  cells  may 
come  together,  and 
the  cytoplasm  of 
the  cells  may  fuse, 
while  the  nuclei 
retain  their  indi- 
viduality. For 
this  to  occur,  it 
appears  necessary 
that  the  cytoplasm 
should  be  in  a  pe- 
culiarly labile  and 
plastic  condition, 
since  instances 
have  been  ob- 
served and  re- 
corded of  one 
Amoeba  swallowing 
another,  and  later 
egesting  it  with 
its  individuality 
unimpaired,  no 
such  fusion  having  taken  place.  In  many  of  the 
Flagellata  (c/.  Bodo,  above)  zygosis  is  preceded  by  an 
"amoeboid"  stage,  which  is  undoubtedly  correlated 
with  a  more  viscid  and  plastic  condition  of  the  proto- 
plasm.     It  is  impossible  that  there  should  not  be 


Fig.  60.  —  Plastogamy  in  a  protozoan 
(Trichosphcerium) .  In  three  places  (marked 
with  a  1)  the  limiting  boundary  of  the 
individuals  is  still  intact.  The  form  is  multi- 
nucleate. —  (From  Lang,  after  Schaudinn.) 


152  GENERAL  BIOLOGY 

some  interaction  between  the  cytoplasm  of  two  fusing 
individuals,  even  if  the  nuclei  remain  individually 
distinct. 

Nuclear  Conjugation  (Karyogamy) . — In  the  great 
majority  of  cases,  however,  both  in  plants  and 
animals,  the  essential  feature  of  zygosis  appears  to 
be  the  fusion  of  the  nuclei  of  the  two  conjugating 
gametes.  Just  as  it  seems  necessary  for  the  cyto- 
plasm to  be  in  a  peculiar  physical  (and  doubtless 
also  chemical)  condition  before  plastogamy  can  take 
place,  so  also  of  the  nucleus.  But  whereas  the  pre- 
liminary nuclear  changes  are  perhaps  more  compli- 
cated and  subtle  than  those  of  the  cytoplasm,  they 
are  easier  to  observe. 

In  all  the  Metazoa  and  Metaphyta,  specialization 
has  progressed  to  the  point  of  differentiation  between 
somatic  cells  and  germ-cells.  The  latter  cells  share 
with  the  former  a  common  ancestry ;  that  is  to  say, 
in  any  one  individual  they  must  have  all  descended 
from  a  single  zygote.  There  is  reason  for  believing, 
however,  that  the  germ-plasm  is  differentiated  from 
the  soma-plasm  at  the  beginning  of  individual 
development.  It  is  therefore  not  a  matter  of  indif- 
ference which  cells  become  gametes.  But  since  the 
individual  itself  must  come  to  a  point  of  sexual 
maturity  before  its  gametes  are  able  to  continue 
the  existence  of  the  race  by  conjugating  with  other 
gametes,  sexual  maturity  of  the  individual  is  coinci- 
dent with  the  maturity  of  its  gametes.  In  other 
words,  in  spite  of  the  fact  that  the  germ-plasm  is  a 


ONTOGENESIS  153 

thing  apart  from  the  soma-plasm,  its  cells,  like  soma- 
cells,  go  through  the  stages  of  progressive  special- 
ization, characteristic  of  individual  development 
(ontogeny).  Of  the  physico-chemical  changes  in- 
volved in  this  specialization  we  have  no  inkling. 
The  external  and  visible  changes  incident  to  its 
conclusion  have  been  carefully  studied,  and  to  them 
is  usually  applied  the  term  maturation.  In  animals, 
the  process  is  practically  the  same,  up  to  the  final 
stage,  in  both  sperm  and  ova.  It  may  be  roughly 
divided  into  three  successive  periods :  a  period  of 
multiplication  of  individual  pro-gametes  (apparently 
without  differentiation),  followed  by  a  period  of 
growth,  very  much  more  extensive  in  the  ovum 
than  in  the  sperm,  and  lastly,  a  period  of  differen- 
tiation which  is  known  as  reduction.  In  many 
species  the  eggs  are  shed  from  the  parental  body 
at  the  conclusion  of  the  second  period.  But  before 
zygosis  can  occur,  reduction  must  always  take  place. 
In  this  remarkable  phenomenon  the  e^g  or  sperm 
divides  twice  in  rapid  succession,  by  mitosis,  —  a 
mitosis,  however,  which  differs  markedly  from  all 
others  known,  in  two  particulars.  In  the  first  place, 
when  the  chromatin  skein  begins  to  condense  and 
resolve  itself  into  chromosomes,  instead  of  the  usual 
number  of  chromosomes  there  appear  in  the  spindle 
of  the  maturing  gamete,  groups  of  four,  known  as 
tetrads,  of  which  there  are  half  as  many  as  the  normal 
number    of    chromosomes.^     The    elements    of    the 

^  It  is  believed  that  the  tetrad  arises  by  a  fusion  of  chromosomes  in 
pairs  (synapsis)  and  the  subsequent  spHtting  of  the  fused  chromosomes. 


154  GENERAL  BIOLOGY 

tetrad  behave  as  chromosomes  in  the  subsequent 
stages  of  mitosis.  Half  of  them  are  pulled  to  one 
pole  of  the  spindle  and  half  to  the  other,  and  the 
cell-wall  that  forms  divides  the  maturing  gamete 
(termed  spermatocyte,  in  the  male,  or  oocyte,  in  the 
female)  in  two.  The  second  peculiarity  of  the  matu- 
ration mitoses  arises  from  the  fact  that  when  these 
two  daughter-cells  (spermatocytes  or  oocytes  of  the 
second  order)  divide  again,  they  do  so  without  the 
intervening  "resting  stage"  which  is  elsewhere 
universal  in  indirect  cell  division.  The  chromatin 
units  have  already  been  segregated'  into  two  groups 
in  the  first  division.  They  are  immediately  divided 
again  into  two  groups,  which  separate  in  the  two 
daughter-cells.  Four  daughter-cells  thus  result 
from  the  division  of  the  first  spermatocyte  (or 
oocyte),  and,  although  the  latter  had,  previously, 
twice  the  normal  number  of  chromatin  units,  the 
number  in  each  of  the  resulting  four  cells  has  been 
reduced  to  half  the  normal  number  on  account  of 
these  two  rapidly  succeeding  divisions.  This  reduc- 
tion of  the  chromosomes  is  of  much  theoretical  inter- 
est in  connection  with  the  subject  of  heredity.  The 
process  seems  to  be  identical  in  both  egg  and  sperm 
so  far  as  the  nucleus  is  concerned,  but  the  dif- 
ference in  the  behavior  of  the  cytoplasm,  in  sper- 
matocyte and  oocyte,  is  very  marked.  The  two 
cleavages,  in  the  case  of  the  spermatocyte,  result 

longitudinally  and  transversely,  to  produce  the  four  units  of  the  tetrad. 
Accordingly,  there  are  half  as  many  tetrads  but  twice  as  many  chromatin 
units  as  the  normal  number  of  chromosomes. 


ONTOGENESIS  155 

in  the  production  of  four  cells  of  equal  size,  eacK  of 
which  metamorphoses  into  a  sperm-cell  character- 
istic of  its  species  and  capable  of  "  fertilizing  "  an 
egg-cell.  But  in  the  oocyte  of  the  first  order,  whereas 
the  cleavage  of  the  nucleus  is  equal  in  each  of  the 
two  daughter-cells  formed,  that  of  the  cytoplasm  is 
so  unequal  that  one  of  the  daughter-cells  is  but  a 
fraction  of  the  size  of  the  other.  In  the  second 
cleavage  the  same  disparity  is  seen,  so  that,  al- 
though as  the  result  of  the  two  successive  cell  divi- 
sions four  cells  result,  one  of  these  is  very  many 
times  the  bulk  of  the  other  three  together.  This 
larger  cell  is  the  one  usually  called  the  egg.  The 
other  three  cells,  which  may  be  looked  upon  as 
abortive  eggs,  are  called  polar  bodies  because  they 
usually  remain  attached  for  a  time  to  the  so-called 
animal  pole  of  the  egg-cell.  They  finally  fall  off 
and  (Jisintegrate.  Like  the  sperm,  the  egg  which 
has  budded  off  its  polar  bodies  has  the  reduced 
number  (one  half)  of  chromosomes. 

Nuclear  Phenomena  of  Zygosis  in  Animals 

Conjugation.  —  The  male  gamete,  or  sperm,  is, 
in  most  animals  and  in  the  lower  aquatic  plants,  a 
highly  specialized  cell  of  minute  size,  equipped  with 
one  or  more  whiplike  flagella  that  enable  it  to  swim 
rapidly  in  a  liquid  medium.  In  the  higher  animals 
the  shape  of  the  sperm  is  often  thg.t  of  a  miniature 
tadpole.  The  egg-cell,  on  the  other  hand,  in  most 
cases'  is  spherical  in  form.  It  is  enormously  greater 
in  volume  than  the  sperm,  its  size  depending  upon 


156  GENERAL  BIOLOGY 

the  amount  of  reserve  food-substance  with  which 
it  is  packed.  In  spite  of  the  disparity  in  size  of  the 
two  kinds  of  germ-cells,  so  far  as  the  nucleus  goes 
both  cells  are  very  much  alike. 

In  animals,  when  a  sperm-cell  encounters  an  egg, 
it  penetrates  the  outer  surface  and  generally  insti- 
gates some  sort  of  a  sudden  change  in  the  egg  cyto- 
plasm that  prevents  other  sperms  from  entering. 
In  some  forms,  however  (e.g.  birds),  it  seems  to  be 
the  rule  for  a  number  of  sperms  to  penetrate  the 
egg.  But  only  one  of  them  functions  in  the  process 
of  zygosis.  In  most  cases  the  tail  is  absorbed 
and  the  nucleus  moves  in  toward  the  center  of  the 
egg-cell,  while  the  egg-nucleus  advances  to  meet 
it,  impelled  by  some  sort  of  "  attraction  "  not  yet 
understood.  When  the  two  nuclei,  or  "  pro- 
nuclei "  as  they  are  called  at  this  time,  have  met, 
they  merge  into  one  nucleus.  The  commingling 
of  chromatin  material  is  probably  not  a  true  fusion, 
inasmuch  as  the  chromosomes  into  which  the  nu- 
cleus is  resolved  appear  to  maintain  their  individ- 
uality in  subsequent  cell  divisions.  From  this  fact 
it  results  that,  since  the  pro-nucleus  of  each  gamete, 
having  undergone  "  reduction,"  has  but  half  the 
number  of  chromosomes  characteristic  of  the  species, 
the  nucleus  of  the  zygote,  composed  as  it  is  of  the 
sum  of  two  such  pro-nuclei,  has  the  normal  number 
of  chromosomes  restored.  In  this  way  the  chromo- 
some number  is  maintained  unchanged,  from  genera- 
tion to  generation.  It  must  also  be  borne  in  mind 
that  since  every  cell  of  the  organism  traces  its  origin 


ONTOGENESIS 


157 


p.6.l-~ 


i».*.^---i 


h 


k 


Fig.  61.  —  Diagrams  illustrating  the  maturation,  zygosis,  and  cleav- 
age of  an  animal  egg.  The  primordial  germ-cell  is  represented  as  possess- 
ing four  chromosomes.  —  (Hegner.) 

— ^  its  cell  genealogy,  so  to  speak — back  to  the  unicel- 
lular-zygote,  and  since  the  chromatin  of  the  nucleus 
of  the  zygote  is  derived,  half  from  one  gamete  and 


158  GENERAL  BIOLOGY 

half  from  the  other,  every  cell  in  the  resultant  organ- 
ism shares  this  double  nature,  that  is,  half  its  chroma- 
tin is  paternal,  half  maternal. 

Following  the  formation  of  the  cleavage  nucleus, 
as  the  nucleus  of  the  unicellular  zygote  is  called,  a 
series  of  rapidly  accomplished  transformations  en- 
sues, which  molds  the  undifferentiated  cell  into  the 
form  characteristic  of  the  species.  These  changes 
may  be  grouped  into  three  stages  : 

First :  A  period  of  rapid  cell  multiplication  un- 
accompanied by  growth. 

Second :  A  period  of  growth,  together  with  — 

Third :  A  period  of  differentiation,  during  which 
the  tissues  resulting  from  the  first  period  become 
changed  from  a  generalized  condition  to  one  involv- 
ing specification  or  specialization. 

These  three  periods  cannot  be  sharply  distin- 
guished, one  from  another,  particularly  the  second 
and  third. 

Cleavage.  —  In  the  first  stage,  just  mentioned, 
the  original  zygote  becomes  subdivided  again  and 
again  into  a  great  number  of  cells,  which,  with  each 
cell  division,  become  smaller  and  smaller.  The 
energy  for  these  repeated  mitoses  is  found  in  the 
reserve  food  material  which  is  stored  up  in  the  egg- 
cell,  even  before  its  maturation.  In  such  animals 
as  have  a  very  brief  larval  period,  or  in  which  the 
larva  is  self-supporting  at  a  very  early  age,  the 
amount  of  such  reserve  food,  or  "  yolk,"  is  much 
less  than  is  required  by  a  form  which  has  a  long 


ONTOGENESIS  159 

period  of  development,  as,  for  instance,  a  bird. 
In  the  latter  case,  the  amount  of  reserve  food  is 
so  great  that  it  bulks  far  more  than  the  proto- 
plasm itself,  and  crowds  the  greater  part  of  the  liv- 
ing substance  to  one  side  of  the  cell.  On  the  other 
hand  some  eggs  contain  so  little  food  yolk  that  they 
are  almost  transparent,  and  what  metaplasm  there 
is  is  evenly  distributed  throughout  the  egg-cell. 
Such  an  egg  is  called  isolecithal.  In  the  eggs  of 
many  animals,  even  before  cleavage  begins,  the  pro- 
toplasm may  be  seen  to  be  visibly  differentiated  in 
different  regions  which  are  destined  to  develop  into 
various  parts  of  the  organism.  There  appears  to 
be  every  gradation  in  the  degree  of  such  differen- 
tiation. In  some  cases  there  is  apparently  none  at 
all.  In  others,  different  zones  or  strata  may  be 
seen.  In  many  such  eggs  mutilation  of  certain  parts 
of  the  egg-cell  produces  corresponding  defects  in  the 
resultant  organism,  showing  that  the  protoplasm  was 
already  specifically  differentiated  before  cleavage. 

The  amount  and  distribution  of  the  food-yolk  is 
of  the  greatest  influence  in  determining  the  nature 
of  the  cleavage  process.  In  an  isolecithal  egg  the 
first  cleavage  usually  divides  the  egg-cell  into  two 
approximately  equal  daughter-cells,  which,  in  turn, 
divide,  each  into  two  others  of  about  the  same  size. 
This  process  is  repeated  in  rapid  succession  until 
the  original  zygote  has  been  halved,  quartered,  and 
then  cut  into  8,  16,  32,  64,  128  cells,  etc.,  the  process 
continuing  until  the  first  step  in  differentiation 
occurs.     The  result   of   such  repeated   cell  division 


160  GENERAL  BIOLOGY 

is  a  rounded  mass  of  tiny  cells  that  looks  somewhat 
like  a  mulberry,  and,  for  that  reason,  the  stage  is 
often  called  the  morula.  In  very  few  species,  how- 
ever, is  the  process  of  cleavage  so  regular  and  sym- 
metrical as  just  described.  In  many  forms,  not 
only  are  the  cells  of  a  different  size,  but  the  cleavages 
do  not  all  occur  in  regular  order,  some  coming  on 
faster  than  others.  In  nearly  all  cases,  however, 
the  result  of  the  continued  cleavage  is  to  produce 
a  mass  of  cells  that  remain  attached  to  one  another 
in  a  single  layer.  Owing  perhaps  to  the  stress  of 
surface  tension  acting  on  the  cells  and  pulling  them 
away  from  the  center,  the  resultant  is  a  hollow  sphere. 
The  stage  just  described  is  called  the  hlastula.  The 
closed  cavity  surrounded  by  the  single  layer  of  cells 
(blastomeres)  is  called  the  blastoccele. 

In  eggs  in  which  there  is  a  large  amount  of  yolk, 
the  cells  resulting  from  cleavage  are  of  very  unequal 
size.  Since  the  yolk  is  heavier  than  the  protoplasm, 
it  "  settles  "  to  one  side  of  the  zygote,  and  the 
blastomeres  that  arise  on  that  side  are  therefore 
larger  on  account  of  the  presence  of  the  yolk,  and, 
for  the  same  reason,  slower  in  forming  than  those 
of  the  opposite  side.  This  reaches  its  extreme  in 
eggs  like  those  of  birds,  in  which  the  protoplasm  is 
crowded  to  a  small  disk-like  spot  at  one  side,  and  the 
mass  of  yolk  is  so  great  that  the  cleavage  is  not  accom- 
plished at  all,  but  affects  only  this  protoplasmic 
disk-like  spot.  Such  an  egg  is  termed  meroblastic 
or  partially  cleaving,  in  contrast  to  holoblastic  or 
totally  cleaving,  eggs. 


ONTOGENESIS 


161 


Gastrulation.  —  Differentiation  may  be  said  to 
begin  at  about  this  point,  although,  as  we  have  seen, 
the  undivided  zygote,  in  many  cases,  is  regionally 
differentiated  from  the  start.  When  the  blastula 
stage  has  been  reached  (in  a  typical  holoblastic 
egg),  one  side  begins  to  pit  in  or  "  invaginate," 
as  one  might  indent  a  soft  rubber  ball  with  his  thumb. 
(See  fig.  62.)     This  process  continues  in  some  species 


Fig.  62.  —  Cleavage  and  gastrulation  in  a  holoblastic  egg  (the  pond 
snail):  a,  undivided  zygote;  6,  first  cleavage;  c,  second  cleavage;  rf,  third 
cleavage;  e,  blastula;  /,  blastula  in  section;  g,  beginning  of  invagination, 
in  section;  h,  completed  gastrula,  in  section. —  (From  Jordan  and  Kel- 
logg, after  Rabl.) 

of  animals  until  the  invaginated  cell-wall  comes  to 
lie  against  the  inner  side  of  the  rest  of  the  blastula 
wall,  obliterating  the  blastoccele  and  forming  a  two- 
layered  shell  surrounding  a  newly  formed  central 
cavity.  This  cavity,  unlike  that  of  the  blastula, 
opens  to  the  exterior  at  the  region  of  invagination. 
This  stage  is  called  the  gastrula,  the  cavity  is  called 
the  archenteron,  and  its  opening  to  the  exterior  just 


162  GENERAL   BIOLOGY 

mentioned,  the  blastopore.  In  many  eggs  the  invag- 
inating  process  stops  before  the  blastocoele  is  oblit- 
erated, but  the  same  relations  of  parts  obtain. 
In  eggs  in  which  yolk  is  very  abundant,  such  as 
those  of  the  frog,  the  yolk-laden  part  of  the  zygote 
cleaves  into  much  larger  cells  (macromeres)  than 
the  opposite  region,  —  so  large,  in  fact,  that  these 
cells  bulk  greater  than  the  segmentation  cavity  of  the 
blastula,  and  it  is  physically  impossible  for  these 
cells  to  invaginate  into  such  a  cavity.  The  same 
result  is  reached,  however,  by  a  growth  of  the  smaller 
upper-layer  cells  over  the  macromeres,  so  that  the 
latter  disappear  within  the  former.  The  final  result 
is  essentially  the  same  as  that  previously  described, 
viz.  a  two-layered  gastrula  with  a  central  cavity 
(archenteron),  communicating  with  the  exterior  by  a 
blastopore. 

Further  Differentiation.^ — In  the  higher  verte- 
brates the  process  of  gastrulation  is  greatly  modified 
and  obscured  by  the  nature  of  the  cleavage,  but 
essentially  the  same  change  takes  place  in  all  animals, 
—  the  conversion  of  a  single-layered  blastula  into  a 
double-layered  gastrula.  The  outer  layer  of  the 
gastrula  is  called  ectoderm,  the  inner  layer,  endoderm. 
There  quickly  develops  between  the  tw^o,  as  an  out- 
growth, mainly  from  the  endoderm,  a  third  layer  or 
mass  of  cells  known  as  the  mesoderm  or  middle 
layer.  From  these  three  germ-layers  there  are  sub- 
sequently differentiated  all  the  tissues  and  organs 
of  the  animal  body.     From  the  ectoderm  develops 


ONTOGENESIS  163 

the  outer  integument,  the  sense  organs,  and  the 
nervous  system.  From  the  endoderm  develops  the 
digestive  portion  of  the  alimentary  canal.  From  the 
mesoderm  develop  all  the  other  structures.  Just 
as  the  unicellular  zygote  became  differentiated  into 
the  blastula,  and  the  latter  into  the  gastrula,  so  the 
germ-layers,  which  are  primarily  sheets  of  apparently 
similar  cells,  differentiate  into  the  most  varied  forms. 
This  is  accomplished  ultimately  by  the  differentiation 
of  the  cells  themselves  (histogenesis),  as  in  the  forma- 
tion of  muscle  cells  or  nerve-cells,  but  such  a  change  is 
preceded  by  a  shaping  of  rudiments  of  organs  by  the 
differential  growth  of  the  germ-layers.  Thus  the 
ectoderm,  in  the  vertebrates,  forms  first  a  shallow 
groove  along  the  axis  of  the  embryo,  which  becomes 
deeper  by  the  more  rapid  growth  of  the  sides,  until 
the  latter  close  over  and  meet  above  to  form  a  tube, 
the  anterior  end  of  which,  by  a  further  complication 
of  folds,  flexures,  and  cell-growth  becomes  the  em- 
bryonic brain.  The  cells  composing  such  a  struc- 
ture are  apparently  all  alike  in  appearance.  Not 
until  the  organ  is  "blocked  out"  does  histogenesis 
begin. 

The  tracing  out  of  the  relative  times  of  appearance, 
the  structure,  and  mutual  relations  of  the  various 
parts  df  the  organism  as  they  are  transformed,  one 
from  another,  as  well  as  the  dynamic  factors  that 
may  control  or  alter  these  changes,  constitutes  the 
special  science  of  embryology.  The  details  of  these 
later  transformations  vary  greatly  in  different  forms 
of  life.     All  animals,   however,  begin  development 


164  GENERAL  BIOLOGY 

with  the  unicellular  zygote,  which,  following  cleavage, 
is  changed  into  a  blastula  that,  in  turn,  is  transformed 
into  a  gastrula.  Both  blastula  and  gastrula  stages, 
particularly  the  latter,  may  be  very  greatly  modified 
by  secondary  conditions.  It  was  recognized  by  one 
of  the  earliest  embryologists  that  the  younger  stages 
of  all  animals  resemble  one  another  much  more 
closely  than  do  later  ones.  This  generalization  may 
be  stated  :  development  is  from  the  general  to  the 
particular,  from  the  relatively  undifferentiated  to  the 
specialized  (Von  Baer's  law).  The  whole  series  of 
transformations  which  an  organism  undergoes  from 
zygote  to  individual  dissolution  is  termed  its  Ontogeny. 

Conjugation  in  the  Protozoa.  —  The  Protozoa, 
in  spite  of  their  apparent  simplicity,  are  in  most 
cases  very  highly  specialized,  and  their  specializa- 
tion is  nowhere  so  marked  as  in  their  methods  of 
sexual  reproduction.  The  majority  of  them  re- 
produce rapidly  under  favorable  circumstances  by 
binary  fission,  or,  in  some  cases,  by  the  formation  of 
spores.  After  a  time,  owing  to  influences  which  are 
imperfectly  understood,  this  asexual  reproduction 
slows  down  and  comes  to  a  standstill.  It  may  be 
artificially  stimulated  (in  the  case  of  ciliates)  by 
adding  chemical  substances  to  the  medium,  such  as 
beef-extract,  potassium  phosphate,  etc.  On  the 
other  hand  if  each  generation  of  the  dividing  ciliates 
is  segregated  in  a  few  drops  of  culture  fluid,  asexual 
reproduction  may  go  on  apparently  indefinitely. 
Experimenters  have  followed  such  a  line  beyond  the 


V.X 


ijOijij 


Fig.  63.  —  Conjugation  and  reproduction  of  Paramecium.  The 
maeronucleus  has  been  omittod  from  all  except  a.  After  attachment 
the  micronuclei  of  each  conjugant  divides  twice  (6,  c)  ;  three  of  the  result- 
ing nuclei  degenerate  and  the  other  divides  again  (c,  d) ;  one  of  the  half- 
nuclei  then  passes  over  to  the  other  cell  and  there  fuses  with  the  one  left 
behip,d  (e,  f)  ;  the  animals  then  separate  ;  the  zygotic  nuclei  then  divide 
repeatedly  (i-m),  some  of  them  transforming  into  macronuclei ;  the 
cells  then  reproduce  by  binary  fission  until  the  nuclei  are  distributed  in 
the  usual  way ;  reproduction  then  takes  place  in  the  ordinary  manner. 
—  (From  Hegner,  after  Calkins  and  Cull.) 


166  GENERAL  BIOLOGY 

three  thousandth  generation.  Whatever  the  causes 
may  be  that  induce  conjugation  in  an  ordinary  cul- 
ture, it  seems  to  be  evident  that  they  are  external  to 
the  organism. 

In  many  of  the  Protozoa,  as  already  described,  con- 
jugation is  a  complete  mingling  of  the  two  organisms 
in  a  zygote.  In  others,  conjugation  is  temporary, 
and  there  is  an  exchange  of  nuclear  substance  between 
the  two  cells.  In  the  ciliate  infusoria,  of  which 
Paramecium  is  the  most  familiar  example,  the 
macronucleus  degenerates  and  the  micronucleus  of 
each  conjugant  divides  twice.  Three  of  the  four 
resulting  nuclei  degenerate.  The  fourth  divides 
again,  one  half  passing  into  the  other  conjugating 
cell  and  there  fusing  with  the  alternate  half  that  does 
not  pass  over.  The  two  individuals  then  separate, 
and  after  a  number  of  divisions  of  the  new,  compound 
nucleus,  a  rapid  series  of  cell  divisions  results  in  the 
production  of  a  great  many  new  individuals. 

Many  of  the  Protozoa  form  spores  under  certain 
circumstances.  This  is  especially  characteristic  of 
the  parasitic  forms.  It  is  very  interesting  to  dis- 
cover that  Amoeba  proteus,  which  is  usually  con- 
sidered one  of  the  simplest  of  all  organisms,  and  the 
vital  processes  of  which  are  frequently  used  to  illus- 
trate the  functions  of  protoplasm  at  their  lowest 
level  of  specialization,  has  a  very  complicated  process 
of  spore-formation.  The  nucleus  of  the  Amoeba 
divides  repeatedly  until  about  seventy  small  nuclei 
result.  These  then  fuse,  two  by  two  (still  within 
the  same  cell),  and  the  fused  nuclei  redivide  several 


ONTOGENESIS  167 

times  until  two  hundred  or  more  result,  each  of  which 
forms  a  "  spore-mother  cell,"  which,  in  turn,  pro- 
duces a  number  of  spores.  From  each  spore  there 
develops  a  new  Amoeba. 

Parthenogenesis.  —  We  have  seen  that  reproduc- 
tion is  a  kind  of  discontinuous  growth,  and  that 
while  it  is  a  usual  accompaniment  of  zygosis  in  the 
higher  forms,  there  is  probably  no  fundamental  con- 
nection between  the  two  phenomena.  Zygosis, 
whether  partial  or  incomplete,  may  perhaps  be  looked 
upon  as  a  sort  of  rejuvenescence  of  the  organism  which 
is  the  more  necessary  and  occurs  the  more  fre- 
quently in  proportion  to  the  degree  of  specialization 
of  the  organism.  In  the  majority  of  the  higher 
forms  it  is  necessary  with  every  individual  generated. 
Some  of  the  Metazoa,  however,  like  the  Protozoa,  are 
able  to  produce  successive  generations  for  a  long 
time  without  the  stimulus  of  sexual  conjugation. 
Such  a  phenomenon  is  known  as  parthenogenesis  or 
agamy,  and  occurs  as  a  normal  incident  of  existence 
in  many  forms,  particularly  the  insects. 

In  the  plant-louse,  or  Aphis,  the  outline  of  the  life 
history  of  a  common  species  is  something  as  follows  : 
in  the  autumn  the  female  lays  a  large  so-called  winter 
egg,  which,  like  most  eggs,  has  been  "  fertilized  "  by 
union  with  the  sperm.  In  the  following  spring  this 
egg  hatches  into  a  wingless  female  called  the  stem- 
mother,  that  forthwith  begins  to  reproduce  partheno- 
genetically  a  long  series  of  generations  like  herself, 
precisely  as  a  single  Paramecium  may  populate  a 


GENERAL  BIOLOGY 


bowl  of  water.  Should  food  fail,  these  females 
develop  wings  and  fly  to  a  more  favorable  locality, 
where  they  in  turn  start  a  series  of  agamic  genera- 
tions. This  is  kept  up  until  fall,  when,  instead  of 
the  " parthenogenetic  females"  there  are  produced 
(still  agamically)  sexual  forms,  which  mate.  The 
female  lays  her  winter  egg,  and  this  provision  for  the 
continuity  of  the  species  having  been  made,  the 
whole  season's  progeny  dies. 

Natural  parthenogenesis  of  this  sort  is  strikingly 
illustrated   in   some  of    the    minute    gall-producing 


Fig.  64.  —  A  gall-making  wasp  (Holcaspis  globulus) :  A,  galls  on  oak, 
natural  size;  B,  the  gall-maker,  twice  natural  size.  —  (From  Folsom's 
"  Entomology,"  permission  of  P.  Blakiston's  Son  &  Co.) 


wasps.  These  insects  lay  their  eggs  in  the  tender 
tissue  of  a  plant,  which  reacts  by  producing  a  tumor- 
like "gall,"  within  which  the  egg  develops  and  from 
which  the  perfect  insect  of  the  second  generation  later 
emerges.  In  many  species  of  these  insects,  only  one 
sex  is  known.  It  has  been  discovered,  however, 
that  in  some  forms  the  insect  that  emerges  from  a 
gall  produced  by  one  "  species  "  is  of   the  type  of 


ONTOGENESIS  169 

another  "  species."  One  type  is  parthenogenetie, 
the  other  sexual,  in  regular  seasonal  alternation. 
Of  some  species,  the  male  is  unknown,  and  it  may  be 
that  the  female  reproduces  continuously  by  partheno- 
genesis. Development  by  zygosis  seems  to  be  only 
an  occasional  incident  to  the  life  history  of  such  a  race. 

Artificial  Parthenogenesis.  —  Although  not  many 
kinds  of  animals  naturally  develop  agamically,  yet 
those  that  do  are  by  no  means  "  lower  types." 
On  the  contrary  they  are  usually  highly  specialized, 
and  the  ability  to  develop  in  this  way  is  an  expression 
of  their  specialization.  Such  examples  demonstrate 
that  whatever  may  be  the  function  of  the  sperm  in 
zygosis,  the  egg  of  each  species  contains  within 
itself  alone  the  potentiality  of  developing  into  an 
individual,  typical  of  the  species  of  which  it  is  a 
member.  Such  being  the  case,  if  development  can 
be  inaugurated  in  an  egg  that  never  normally  develops 
by  zygosis,  by  analyzing  the  agency  that  brings 
about  such  an  effect,  w^e  may  get  a  clue  to  the  nature 
of  the  action  of  the  sperm  in  producing  the  same 
result.  Within  the  last  few  years  a  great  deal  of 
experimental  work  has  been  done,  which,  although 
perhaps  it  does  not  get  very  far  in  analyzing  the 
phenomena,  does  serve  to  show  the  subtle  complexity 
of  the  forces  involved,  and  indicates  the  nature  of  the 
stimulus  exerted. 

li  was  found  first  that  potassium  chloride  added  to 
sea  water  in  which  were  the  matured  eggs  of  a  sea- 
worm,  Chcetopterus,  induced  the  egg   to   undergo  a 


170  GENERAL  BIOLOGY 

rapid  series  of  divisions,  producing  a  cell-mass  which 
roughly  resembled  a  larva.  This  was,  however, 
without  cellular  organization,  and  eventually  werlt 
to  pieces.  Such  cell  development  at  random  is 
suggestive  of  the  condition  we  find  in  tumors  and 
galls,  —  reproduction  without  organization.^  In  the 
case  of  Choetopterus,  however,  differentiation  may 
take  place  even  without  cellular  organization. 

It  was  found  that  the  eggs  of  other  animals,^  when 
ripe,  can  be  induced  to  segment  by  a  variety  of 
means,  —  shaking,  varying  the  osmotic  pressure 
of  the  sea-water,  acids,  CO2,  etc.,  all  being  effective, 
though  not  always  for  the  same  species. 

Continuing  these  experiments,  a  number  of  Ameri- 
can investigators,  among  whom  Professor  Jacques 
Loeb  is  the  most  conspicuous,  have  succeeded  in 
imitating,  artificially,  the  processes  of  Nature,  by 
inducing  unfertilized  eggs  to  develop  normally  by 
chemical  and  physical  means.  The  methods  em- 
ployed have  been  complicated,  and  the  data  are  as 
yet  too  incomplete  to  afford  a  basis  for  generalization, 
but  such  experiments  show  that  probably  all  eggs 
contain  within  themselves  all  the  potentialities  for 
normal  development,  and  that  "  fertilization  "  or 
zygosis  is  only  an  accompaniment  to,  not  a  necessity 
for,  individual  development.^ 

1  Mature  eggs  that  are  not  fertilized  quickly  disintegrate  (autolysis). 
The  potassium  prolonged  the  life  of  the  protoplasm  intheabove  experiment. 

2  The  sea-urchin  is  a  favorite  form  for  experiment. 

3  In  spite  of  the  truth  of  this  statement  we  must  not  lose  sight  of  the 
profound  significance  of  the  phenomenon  of  zygosis  from  another  stand- 
point.    The  mixture  of  germinal  substance  (amphimixis)  that  results 


ONTOGENESIS  171 

Like  a  wound  clock  (to  vary  Preyer's  comparison), 
the  egg  is  a  mass  of  matter,  of  which  the  parts, 
although  in  unstable  equilibrium,  are  at  rest  because 
of  a  sort  of  inertia.  When  an  appropriate  "  stimulus  " 
comes,  whether  that  of  the  sperm  or  that  of  some 
chemical  or  physical  agent,  cell  division  begins  to 
follow  cell  division  in  rapid  succession.  "  Stim- 
ulus "  is  used,  of  course,  in  a  very  broad  sense, 
since  the  changes  induced  in  the  egg-cytoplasm  by 
the  sperm  or  by  external  agents  are  fundamental  in 
character  and  profoundly  alter  the  chemical  and 
physical  nature  of  the  protoplasm.  Like  chemical 
reactions  in  general,  the  rate  of  division  is  influ- 
enced by  the  temperature.  A  regular  rhythm  of 
0-production  and  C02-production  has  been  demon- 
strated in  cleaving  eggs,  which  probably  is  an  external 
indication  of  the  growth  of  the  chromatin  from  the 
cytoplasm  (by  oxidation) ,  in  the  resting  stage  between 
one  mitosis  and  the  next. 

Doubtless  in  both  naturally  parthenogenetic  eggs 
and  ordinary  eggs  much  the  same  sort  of  specific 
stimulus  is  necessary  to  inaugurate  development. 
The  parthenogenetic  individual,  however,  is  able  to 
supply  that  stimulus  itself,  whereas  in  the  majority 
of  animals  it  must  be  supplied  from  without. 

Alternation  of  Generations  in  Animals.  —  Li  the 
worms  that  reproduce  by  fission,  after  a  number  of 

from  the  combination  of  two  gametes  of  diverse  origin  must  bring  about 
a  very  diJBferent  end-result  from  that  which  would  be  the  fate  of  the  egg 
if  it  were  to  develop  by  itself,  i.e.  it  insures  a  bi-parental  inheritance 
(see  Chapter  VII) . 


172 


GENERAL  BIOLOGY 


Fig.  65.  —  A  colonial  Hydroid  (Bougainvillea)  and  its  Hydromedusa  : 
A,  complete  colony,  natural  size ;  B,  a  portion  of  the  same,  magnified, 
showing  the  branched  stem  bearing  hydranths  (hyd.)  and  medusae  (med.), 
one  of  the  latter  nearly  mature,  the  others  undeveloped  :  each  hydranth 
has  a  circlet  of  tentacles  (t)  surrounding  the  mouth  (hyp.),  and  contains 
an  enteric  cavity  (ent.  cav.)  continuous  with  a  narrow  canal  in  the  stem. 
The  stem  is  covered  with  a  cuticle  (cu.) ;  C,  a  medusa  after  liberation 
from  the  colony,  showing  the  bell  with  tentacles  (0  ;  oc,  eye-spots.  — 
(From  Parker,  after  Allman.) 


ONTOGENESIS  173 

individuals  have  been  produced  asexually,  the  latter 
generations  produce  eggs  and  sperm.  The  individuals 
developing  from  the  zygosis  of  these  gametes  again 
reproduce  asexually,  and  so  on.  It  is  the  same  with 
the  majority  of  those  forms  that  normally  reproduce 
by  budding,  fin  a  colonial  hydroid,  for  example, 
the  individuals  of  the  colony  (hydranths)  come  into 
existence  by  the  process  of  budding  from  other 
hydranths  of  the  colony,  and  the  majority  resemble 
their  immediate  progenitors.  But  some  of  the 
individuals  thus  budded  off  differ  from  the  rest  both 
in  structure  and  function.  Instead  of  remaining 
attached  to  the  parent  stem,  they  break  away  as 
free  individuals,  and  their  peculiarly  differentiated 
structure  enables  them  to  live  an  active  existence  in 
the  water.  These  are  the  hydromedusae.  In  the 
second  place  these  individuals,  unlike  the  fixed 
members  of  the  colony,  produce  eggs  and  sperm- 
cells  that  are  released  when  mature  and  conjugate  in 
the  open  sea.  From  the  zygote  thus  formed  there 
develops  afi  individual  that  is  unlike  the  free-swim- 
ming, sexual  medusa,  its  immediate  ancestor,  but 
resembles  the  hydranth  from  which  the  medusa 
budded  off.  By  repeated  asexual  reproduction  this 
produces  another  hydroid  colony.  In  some  fresh- 
water hydras  there  has  been  described  the  develop- 
ment of  sexual  organs  on  buds  still  attached  to  the 
parent  stem.  If  the  habit  of  always  developing  them 
in  such  a  fashion  should  become  confirmed,  it  is  easy 
to  see  how  the  condition  found  in  the  colonial  hydroids 
may  have  come  about.     If  some  of  the  buds  remained 


174  GENERAL  BIOLOGY 

attached  (as  in  the  ordinary  colony),  while  others 
separated  from  the  parent  stock,  and  if  the  function 
of  reproduction  should  have  become  confined  to  these 
latter  colonies,  we  would  have  the  condition  just 
described  for  the  hydroid.  Such  a  division  of  labor 
between  the  parts  of  the  colony  {i.e.  the  individuals), 
whereby  one  particularly  specialized  group  of  in- 
dividuals reproduces  sexually  and  the  rest  asexually, 
is  known  as  metagenesis  or  alternation  of  generations, 
since  the  sexual  individual  resembles,  not  its  im- 
mediate parent  nor  its  descendant,  but,  so  to  speak, 
its  grandparent  or  grandchildren.  The  asexual 
stage  may,  however,  inckide  more  than  one  individ- 
ual, particularly  in  those  types  that  reproduce  by 
fission  or  budding.  Such  an  alternation  of  sexual 
and  asexual  generations  occurs  in  widely  separated 
groups  of  animals,  probably  having  arisen  independ- 
ently in  all  of  them. 

Sexual  Reproduction  in  Plants 

In  the  simplest  plants,  as  in  the  simplest  animals, 
vegetative  or  asexual  reproduction  is  the  rule,  with  or 
without  spore-formation.  In  the  bacteria,  and  some 
green  algae,  no  other  method  is  known.  Of  the  lower 
plants  that  live  in  water,  the  spores  are  frequently 
provided  with  flagella  and  are  motile.  For  this 
reason  they  are  called  zoospores.  The  spores  of 
land-plants  are,  however,  non-motile,  although  in 
some  groups  they  are  so  minute  and  light  that  they 
float  in  the  air  and  are  borne  everywhere  by  the  wind. 


ONTOGENESIS  175 

Although  plants,  in  comparison  with  animals,  are 
handicapped  in  their  ability  to  move  about  over  the 
surface  of  the  earth  and  thus  effect  the  maximum  dis- 
persal of  the  species,  yet  such  a  defect  is  compensated 
by  this  ability  to  produce  spores  in  enormous  quanti- 
ties. But  in  all  except  the  very  simplest  plants 
reproduction  by  germ-cells  is  also  to  be  found.  In 
the  plant  world,  accordingly,  we  find  an  alternation  of 
sexual  and  asexual  generations.  One  plant  (genera- 
tion) produces  gametes  (whence  it  is  called  the 
gametophyte) ;  the  zygote  arising  from  the  fusion  of 
two  gametes  develops  into  another  plant  (genera- 
tion) which  produces  spores  and  is  therefore  called 
the  sporophyte.  The  gametophyte  generation  in 
many  groups  is  telescoped  into  the  sporophyte,  as  it 
were,  and  its  true  relations  can  be  made  clear  only 
in  comparison  with  simpler  types. 

Liverworts  and  Mosses.  —  In  the  liverworts  the 
plant-body  (thallus)  has  the  form  of  a  green,  leaflike 
structure,  growing  close  to  the  ground,  and  sending 
down  minute,  feeding  root-hairs  into  the  earth 
from  the  lower  surface.  This  is  the  gametophyte. 
Along  the  edges  are  developed  spermaries  (anther- 
idia)  and  ovaries  (archegonia) ,  which  produce  the 
male  and  female  gametes.  The  former  are  active, 
and,  swimming  about  in  the  dew  or  rain,  meet  and 
fuse  with  the  egg-cells.  From  the  zygote  thus 
formed  there  arises,  by  repeated  cell  divisions,  a 
mass  of  cells  within  the  archegonium  itself,  which 
becomes  differentiated  into  a  structure  quite  unlike 


176  GENERAL  BIOLOGY 

the  liverwort  thallus.  This  is  the  sporophyte.  It 
develops  at  the  expense  of  the  archegonial  tissue  and 
matures  within  itself  great  numbers  of  minute 
spores.  When  these  sprout,  they  grow  into  a  plant- 
body  resembling  the  original  "  liverwort,"  —  that 
is,  the  gametophyte. 

Much  the  same  sort  of  development  is  to  be 
observed  in  the  mosses.  The  sporophyte  here  grows 
into  a  long  stalk  (drawing  upon  the  tissues  of  the 
gametophyte  for  its  sustenance),  at  the  end  of  which 
is  borne  the  sporangium.  From  each  of  the  multi- 
tude of  tiny  spores  sprouts  another  gametophyte 
or  moss-plant,  and  so  the  cycle  is  completed.  The 
sporophyte  of  mosses,  unlike  that  of  liverworts, 
although  mainly  dependent  upon  the  gametophyte 
tissue  for  its  food  supply,  is,  nevertheless,  provided 
with  some  chlorophyll-tissue,  and  hence  can  manu- 
facture some  food  of  its  own. 

Ferns.  —  In  the  fern,  the  familiar  leaflike  plant 
is  the  sporophyte ;  the  gametophyte  is  a  tiny,  heart- 
shaped  thallus,  somewhat  like  a  very  simple  liverwort. 
The  gametes  of  both  sexes  develop  on  the  lower 
side  of  the  thallus  (prothallium) .  The  sperm  are 
motile  and  swim  to  the  egg-cell  in  the  dew  or  rain. 
The  fertilized  egg  divides  into  a  mass  of  cells  within 
the  antheridium,  drawing  its  sustenance,  as  in  the 
mosses,  from  the  tissue  of  the  gametophyte.  How- 
ever, it  soon  begins  to  develop  a  green,  leaflike 
branch,  that  grows  upward,  and  a  root-stalk  that 
grows  into  the  earth.     Thereupon  it  is  independent  of 


ONTOGENESIS  177 

the  gametophyte  and  grows  to  a  much  greater  size, 
leaf  after  leaf  being  developed  on  the  growing  root- 
stalk.  Eventually,  sporangia  are  developed  in  enor- 
mous numbers  on  the  under  side  of  the  leaves,  usually 
in  clusters  (sori).  The  spores,  falling  on  damp  soil, 
sprout,  and,  cell  division  following  cell  division, 
a  mass  of  cells  results  which  soon  takes  the  form  of  a 
tiny  prothallium,  —  the  gametophyte  of  another 
generation.  In  the  more  complex  ferns  two  sorts  of 
spores  are  formed,  large  ones  called  megaspores  and 
smaller  ones  known  as  microspores.  Both  produce 
gametophytes  upon  germination,  but  the  prothallium 
arising  from  a  megaspore  produces  female  gametes 
only,  whereas  one  arising  from  a  microspore  produces 
male  gametes  only.  We  have  here  a  sexual  differ- 
entiation in  the  sporophyte  or  non-sexual  phase. 

Seed  Plants.  —  We  have  traced  a  progressive  em- 
phasis which  the  course  of  evolution  has  laid  on 
the  sporophyte,  compared  with  the  gametophyte, 
generation.  In  the  liverwort  the  gametophyte  is 
the  **  plant  "  and  the  sporophyte  a  tiny  parasite 
upon  it.  In  the  higher  mosses  the  gametophyte 
is  larger  and  the  sporophyte,  although  still  largely 
dependent  upon  it,  is  able  to  contribute  some  of  its 
own  food.  In  the  ferns  the  sporophyte,  although 
dependent  upon  the  gametophyte  in  the  beginning, 
soon  shifts  for  itself  and  completely  overshadows  the 
latter.  Finally,  in  the  seed  plants,  the  representa- 
tives^ of  the  plant  world  most  familiar  to  us,  the  condi- 
tions are  reversed,  and  we  find  that  the  sporophyte  is 


178  GENERAL  BIOLOGY 

the  "  plant,"  the  gametophyte  being  a  minute  de- 
pendant upon  it. 

In  the  seed  plants  there  always  occur  the  two 
kinds  of  spores,  —  megaspores  and  microspores. 
The  latter  are  more  often  known  by  the  more  familiar 
term  pollen.  In  some  species  of  plants  both  are 
borne  on  the  same  plant  (monoecious  type) ;  in  others, 
each  is  developed  on  a  different  plant  (dioecious  type). 

The  leaves  which  bear  the  microsporangia  are 
called  stamens,  those  that  bear  the  megaspores, 
carpels.  Both  these  forms  of  metamorphosed  leaves 
are  usually  surrounded  by  other,  highly  modified, 
frequently  colored  leaves  (petals,  sepals),  to  form  a 
flower.  When  the  carpels  grow  together  in  a  mass, 
as  is  frequently  the  case,  we  speak  of  it  as  the  pistil. 

The  carpels  bear  sporangia,  to  which  the  name 
ovule  has  long  been  given,  although  it  must  not  be 
forgotten  that  they  are  not  "  eggs,"  but  are  developed 
by  the  sporophyte,  the  asexual  generation.  The 
megaspore  is  never  released  from  the  sporangium, 
and  the  sexual  generation  begins  its  existence  there. 
On  the  other  hand,  the  microspores  (pollen)  are 
matured  in  great  numbers  in  the  sporangia  of  the 
stamens,  and,  when  released,  are  carried  by  the  wind, 
insects,  and  other  agents  to  the  sticky  termination  of 
the  pistil  (the  stigma).  When  one  sticks  here,  it 
begins  to  sprout  and  grow,  much  as  moss-spore  on 
damp  ground.  Since,  in  the  flowering  plant,  however, 
the  megaspores  are  inside  the  carpels  and  are  never 
released,  the  pollen  spores  must  grow  down  into  the 
carpel  to  them.     This  it  does  in  the  form  of  a  (rela- 


ONTOGENESIS  179 

tively)  long  pollen-tube.  The  pollen-tube  is  the  male 
gametophyte,  and  it  grows  directly  through  the 
tissues  of  the  stigma  and  style,  to  the  megasporan- 
gium,  which  it  penetrates.  One  of  its  cells  (or 
nuclei)  is  the  male  gamete. 

The  Germination  of  the  Megaspore.  —  The  first 
step  in  the  development  of  the  megaspore  consists 
in  the  division  of  its  single  nucleus  into  two,  which 
move  apart  into  either  end  of  the  cell.  Each  nucleus 
then  divides  twice,  so  that  there  are  produced  two 
groups  of  four  nuclei  each,  in  either  end  of  the  spore- 
cell.  The  latter,  meanwhile,  grows  rapidly  at  the 
expense  of  the  surrounding  tissues.  Now,  one 
nucleus  from  each  group  (the  polar  nuclei)  leaves 
the  others  and  moves  toward  the  center  of  the  cell, 
where  the  two  meet  and  fuse.  The  cell  now  contains 
three  nuclei  at  each  end  and  one  double  one  at  the 
center.  The  latter  forms  the  endosperm,  a  mass  of 
food-storing  cells  that  fills  the  embryo-sac.  The 
three  nuclei  at  the  bottom  of  the  cell  (the  antipodal 
nuclei)  disintegrate.  Sometimes  they  form  cell- 
walls  and  even  build  tissue,  but  have  no  further  fate. 
Of  the  other  three  nuclei  at  the  distal  end  of  the  sac, 
one  is  the  egg  or  gamete,  the  other  two  (known  as 
synergids  or  "  helping  cells  ")  are  sacrificed  for  the 
nourishment  of  the  gamete,  much  as  are  the  two 
polar  bodies  of  the  animal-egg. 

The  Germination  of  the  Microspore. — Each  of 
the  microspores  is  functional.  In  germination  the 
nucleus  divides,  forming  two  cells,  a  large  one  called 


180 


GENERAL  BIOLOGY 


Fig.  66.  —  Diagrammatic  Summary  of  the  Life-cycle  of  a  Seed  Plant 
(Alisma).  A-G;  A,  M,  N;  X,  Y,  Z,  A,  —  the  sporophyte  phase;  H-X; 
P-X,  —  the  gametophyte  phase.  A,  the  mature  plant  whose  flowers 
bear  stamens  (B,  C)  and  pistils  (M) ;  N,  a  section  through  the  mega- 
sporangium  ;  0-R,  the  developing  megaspore ;  Q,  with  the  degenerating 
tapetal  (nourishing)  cell.  There  is  but  one  megaspore  in  Alisma  instead 
of  the  four  that  are  found  in  most  seed  plants.  T,  the  complete  embryo- 
sac  with  the  two  endosperm  nuclei  in  the  middle,  the  three  antipodal 
nuclei  at  the  lower  end,  and  the  egg-nucleus  with  the  two  synergids  at  the 
other  end ;  V,  the  egg-nucleus.  D,  the  development  of  the  pollen- 
mother-cells  ;  E,  one  of  the  pollen-mother-cells  that  divides  into  F,  a 
tetrad ;  G,  a  microspore  (pollen  grain)  ;  H,  division  of  the  microspore 
nucleus ;  I,  beginning  of  the  pollen-tube,  with  one  tube-nucleus  and  the 
two  gametic  nuclei ;  W,  fusion  of  male  pronucleus  with  egg-pronucleus ; 
X,  the  zygote  (gametospore)  ;  Y,  beginning  of  embryo  formation  ;  Z,  the 
seed,  containing  the  developed  embryo  which  grows  into  A.  In  some 
forms  the  second  male  pronucleus  K^  fuses  with  the  endosperm  nuclei 
(L).     (After  Schaffner.) 


ONTOGENESIS  181 

the  tube-cell,  and  a  smaller  one  which  divides  again 
into  two  male  gametes.  The  tube-cell,  in  its  growth 
down  the  style,  clears  the  way  for  the  two  gametes, 
and  when  it  has  entered  the  embryo-sac,  it  swells 
and  ruptures,  discharging  the  two  gametes,  one  of 
which  conjugates  with  the  egg-cell,  the  other  with 
the  endosperm  cell. 

The  zygote,  formed  by  the  fusion  of  these  two 
gametes,  develops  into  the  embryo  of  the  plant. 
Meanwhile,  the  endosperm  cell  also  divides  and 
forms  a  growing  tissue  that  packs  about  the  embryo. 
The  food  (oil,  starch,  etc.)  contained  in  the  endo- 
sperm nourishes  the  embryo  plant  until  it  is  able  to 
get  its  own  food.  Finally,  the  outer  layers  of  the 
ovule  secrete  various  kinds  of  hard  protective  cover- 
ings which  form  the  seed-coats,  and  the  young  plant, 
thus  wrapped  up,  is  cast  off  as  a  seed.  In  this  way, 
unfavorable  seasons  are  tided  over.  In  some  cases 
the  carpels  themselves,  or  tissues  adjacent,  become 
soft  and  succulent,  forming  fruits  that  inclose  the 
seed.  When  suitable  conditions  of  temperature  and 
moisture  again  intervene,  the  sporophyte  within  the 
seed  resumes  its  growth  at  the  expense  of  the  stored- 
up  food,  and,  bursting  its  seed-coats,  grows  out, 
takes  root,  and  resumes  the  cycle.  The  embryo 
has  a  root,  a  stem,  and  one  or  two  leaves  called 
cotyledons.  In  those  seed-plants  with  one  cotyle- 
don —  the  monocotyledons  (palms,  lilies,  orchids, 
etc.)  —  the  leaves  are  parallel-veined,  there  is  a 
"  pithy "  stalk  (like  that  of  corn),  and,  usually, 
flower-parts  arranged  in  "  threes."     In  those  with 


182  GENERAL  BIOLOGY 

two  cotyledons  —  the  dicotyledons  —  the  leaves 
are  netted-veined,  the  stem  has  a  hollow  cylinder  of 
wood,  and  the  flowers  are  usually  in  "  fives  "  or 
"  fours." 

In  plants,  as  in  animals,  the  number  of  chromo- 
somes is  constant  for  any  one  species.  When  the 
germ-tissue  of  the  sporophyte  develops  the  spores, 
the  cells  which  are  to  become  megaspores  and 
microspores  are  found  to  have  but  half  the  number 
of  chromosomes  that  occurs  in  other  cells  of  the 
sporophyte.  This  reduction  is  accomplished,  not 
by  the  formation  of  chromatin  "  tetrads,"  as  in 
animals,  but  by  a  precocious  splitting  of  the  spireme 
thread  in  the  mitosis  which  precedes  the  formation 
of  the  spore-mother  cell.  Four  megaspores  are 
formed,  which  are  usually  arranged  in  a  linear  row. 
Likewise,  four  microspores  are  formed,  which  are 
arranged  in  a  spherical  mass  and  called  by  the 
botanists  "  tetrads."  These  tetrads,  composed  each 
of  four  adherent  pollen  grains,  are  to  be  carefully 
distinguished  from  the  chromatin  figures  in  the 
spermatocytes  and  oocytes  of  animals. 

Parthenogenesis  in  Plants.  —  As  in  animals,  so 
in  some  seed  plants,  development  may  take  place 
without  zygosis,  a  phenomenon  that  has  been  re- 
ferred to  previously  as  parthenogenesis.^  Develop- 
ment occurs  by  the  spontaneous  cleavage  of  the  egg- 
cell  and  the  consequent  formation  of  the  embryonic 

1  The  authentic  cases  are  Thallictrum  (two  species),  Alchemilla 
(nearly  all  species),  Ficus  hirta,  and  Antennaris  alpina.  Also  one  pine, 
Pinus  pinaster. 


ONTOGENESIS  183 

plant! .  In  such  cases  there  is  no  reduction  of  the 
number  of  chromosomes.  There  is  evidence  that 
the  initiatory  "  stimulus  "  of  development  may  be 
derived  from  contact  of  the  egg  with  the  surrounding 
endosperm.  In  the  case  of  the  fig  (Ficus),  the 
puncture  made  by  a  tiny  wasp^  (Blastophaga)  is 
probably  the  cause  of  development. 

Apogamy.  —  Very  similar  to  true  parthenogenesis, 
and  much  more  common,  is  the  production  of  a 
sporophyte  by  a  gametophyte  from  other  sources 
than  the  egg-cell,  but  still  without  fertilization. 
This  is  termed  apogamy.  Sometimes  the  embryo 
is  produced  by  growth  of  gametophyte  tissue  (fre- 
quently in  ferns),  and  this  is  to  be  classed  as  simple 
budding.  In  other  cases,  the  embryo  develops  by 
the  cleavage  of  the  synergids,  which  are  to  be  con- 
sidered as  abortive  eggs.  This  is  common  in  the 
orange,  the  dandelion,  and  many  conifers.  In  a 
very  similar  way,  among  the  ferns,  prothallia 
(gametophytes)  may  be  developed  directly  from  the 
leaf  like  sporophyte,  without  the  intervention  of  a 
spore.     This  is  called  apospory. 

The  Probable  Evolution  of  the  Plant  World.  —  The 
simplest  forms,  both  in  animal  and  plant  life,  are 
aquatic,  and  life  appears  to  have  begun  in  the  water. 
In  an  aqueous  medium,  free-swimming  organisms 
can  go  in  any  direction,  and  the  conjugation  of 
gametes  is  effected  with  relative  ease  and  certainty. 

^  It  is  of  interest  to  note  that  in  the  frog's  egg,  development  may  be 
induced  by  puncturing  the  unfertilized  egg  with  a  needle. 


184  GENERAL  BIOLOGY 

The  advantage  of  asexual  reproduction  or  spore- 
formation  lies  in  the  fact  that  species  may  thus  tide 
over  periods  of  unfavorable  conditions,  or,  on  the 
other  hand,  rapidly  multiply  the  vegetative  phase  of 
the  plant's  life  in  favorable  circumstances.  In 
leaving  the  water  for  the  land,  the  original  aquatic 
traits  were  at  first,  in  large  measure,  retained; 
that  is,  the  gametophyte  phase  was  most  prominent, 
and  the  gametes  were  aquatic.  This  is  a  condition 
we  find  in  the  mosses  and  liverworts.  In  the  ferns  we 
still  find  the  gametes  to  be  motile,  aquatic  cells,  but 
the  difficulties  and  dangers  of  this  mode  of  reproduc- 
tion are  compensated  by  a  marked  increase  in  the 
degree  of  specialization  which  the  asexual  or  spore- 
forming  phase  has  attained.^ 

Finally,  in  the  seed  plants  we  find  the  free-swimming 
germ-cells  replaced  by  gametes  that  are  throughout 
protected  and  inclosed  by  the  tissues  of  the  gameto- 
phyte, and  the  young  sporophyte  that  results  from 
their  conjugation  is  protected  and  supplied  with 
food.  Division  of  labor  has  brought  about  an  in- 
creasing efficiency  from  the  standpoint  of  competi- 
tion with  other  types.  The  seed  plants,  independent 
of  water  for  the  purpose  of  zygosis,  and  adapted  to 
secure  the  greatest  protection  for  the  developing 
sporophyte,  as  well  as  for  its  maximum  dispersal, 
have   a   very   great   advantage   over   the    "  lower " 

1  The  advantage  of  asexual  reproduction  lies  in  the  fact  that  the 
species  is  much  less  likely  to  be  exterminated  if  the  vegetative  phase 
of  the  plant's  life  is  emphasized  and  a  wide  dispersal  secured.  More- 
over, the  formation  of  spores  enables  the  species  to  tide  over  periods  of 
unfavorable  conditions. 


ONTOGENESIS  185 

forms,  and  as  a  consequence  we  find  them  the  domi- 
nant type  to-day. 

Morphogenesis 

The  goal  of  the  reproductive  process  is  the  forma- 
tion of  a  new  individual.  In  asexual  reproduction, 
particularly  by  budding,  it  is  often  a  matter  of  great 
difficulty,  and,  perhaps,  of  minor  importance,  to 
determine  where  the  limits  of  individuality  lie. 
In  those  forms  which  develop  sexually,  the  individual 
comes  into  existence  with  the  fusion  of  the  gametes 
(or  of  their  nuclei)  to  form  the  zygote.  In  other 
words,  the  zygote  is  the  new  individual,  however 
little  it  may  resemble  what  we  are  accustomed  to 
call  the  specific  type.  Before  it  can  reproduce  a 
second  generation,  however,  it  must  be  transformed 
into  that  type.  This  it  does  by  a  series  of  remark- 
able changes  (see  page  158  ff.)  accompanied  by 
growth,  to  which  the  name  Morphogenesis  is  given. 

Regeneration.  —  The  phenomenon  of  morpho- 
genesis is  not  only  discoverable  in  the  acquisition 
of  the  specific  form,  that  is,  in  development,  but  also 
in  the  restitution  of  that  form  when  it  is  altered  or 
destroyed.  Thus,  if  one  cuts  off  the  leg  of  a  sala- 
mander or  of  a  crayfish,  a  new  growth  of  tissue  will 
take  place  at  the  cut  surface,  and  this  new  tissue 
will  differentiate  into  a  new  leg  which  is  the  du- 
plicate of  the  one  destroyed,  or,  if  not  the  exact 
duplicate,  at  least  conforms  exactly  to  the  specific 
type.  This  function  of  the  organism  is  known  as 
regeneration.     It  is  not  found  in  all  organisms,  but 


186 


GENERAL  BIOLOGY 


is  particularly  characteristic  of  the  "  lower,"  i.e. 
less  specialized  types.  For  example,  although  the 
leg  of  a  salamander  will  regenerate  as  described, 
as  will  also  that  of  a  frog  tadpole,  that  of  a  mature 
frog  will  not.  Regeneration  is  also  found  in  plants. 
A  leaf  of  Begonia,  if  put  in  water,  will  grow  roots  and 
regenerate  the  whole  plant.  From  one  point  of 
view    there  is   nothing  fundamentally   different    in 


Fig.  67.  —  Regeneration  in  Hydra:  A,  normal  Hydra  (lines  show 
where  piece  was  cut  out) ;  B,  1-4,  changes  in  a  piece  of  A  as  seen  from 
side ;  C,  1-4,  same  as  seen  from  end ;  D,  E,  F,  later  changes  in  same 
piece.  —  (From  Jordan  and  Kellogg,  after  Morgan.) 


the  differentiation  of  a  tissue-fragment  artificially 
sundered  from  the  organism  into  a  new  organism, 
and  the  similar  differentiation  of  a  tissue-mass 
(bud)  naturally  sundered  from  a  parent  organ- 
ism;  or,  indeed,  of  a  single  cell,  whether  spore  or 
gamete,  thrown  off  from  such  an  organism.  In 
each  case  there  is  the  achievement  of  a  certain  spe- 
cific  type,   by   differentiation   from   an   apparently 


ONTOGENESIS 


187 


undifferentiated  mass  of  living  matter,  derived  from 
an  organism  which  also  conforms  to  that  specific  type. 

Regulation.  —  Redifferentiation,  in  the  case  of 
injury  or  alteration  of  form,  is  not  always  accom- 
panied by  growth  of  new  tissue.  If  a  Hydra  be 
cut  in  two,  each  half  will  transform  into  a  complete 


^^1 

11 « 

Fig.  68. — •Regeneration  of  Stentor:  A,  cut  in  three  pieces;  B,  row 
showing  regeneration  of  the  anterior  piece ;  C,  regeneration  of  middle 
piece;  D,  that  of  posterior  piece.  —  (From  Jordan  and  Kellogg,  after 
Morgan.) 

Hydra,  half  the  original  size.  Here  there  has  been 
no  new  growth,  but  a  readjustment  and  shifting 
of  mutual  relations  of  the  old  tissues  of  the  animal. 
Among  the  larger  Protozoa,  such  as  Stentor,  the  same 
thing  occurs  if  the  cell  be  cut  in  fragments.  It  is 
necessary,  however,  for  each  fragment  to  contain  a 
portion  of  the  original  nucleus  in  order  that  the  trans- 


188 


GENERAL  BIOLOGY 


formation  into  a  typical  protozoan  may  take  place. 
Such  a  transformation  of  the  old  tissue,  unaccom- 
panied by  growth,  is  called  regulation.  A  striking 
example  is  found  in  the  reaction  of  young  embryonic 
stages.     If,    for    example,    the    blastula    of    a    sea- 


FiG.  69.  —  Regeneration  of  the  blastuia  and  gastrulse  of  sea-urchins ; 
the  line  indicates  where  the  blastula  or  gastrula  was  cut  in  half ;  the 
smaller  figures  show  results  of  the  regeneration  (regulation)  of  the  two 
halves  of  each.  —  (From  Jordan  and  Kellogg.) 


urchin  be  cut  in  two,  each  half-sphere  will  close  over, 
round  up,  and  form  a  perfect  blastula,  half  the  original 
size.  Each  of  these  goes  through  its  normal  trans- 
formations, and  the  end-result  is  two  individuals 
instead  of  the  single  one  that  began  its  course  of 
development. 


ONTOGENESIS  189 

In  certain  species  a  similar  sequence  of  events 
occurs  as  a  natural  incident  of  development.  Some 
forms  of  parasitic  wasps  lay  eggs  within  the  bodies 
of  caterpillars,  the  young  larvae  feeding  on  the  cater- 
pillar tissues,  and  finally  emerging  to  spin  cocoons  on 
the  surface,  within  which  they  carry  out  their  final 
metamorphosis.  It  has  been  discovered  that  only 
one  egg  may  be  laid  thus  within  the  caterpillar,  but 
that  it  fragments  into  scores  or  hundreds  of  por- 
tions, from  each  of  which  a  perfect  parasitic  wasp 
develops.  This  is  known  as  poly emhry any.  To 
recur  again  to  the  sea-urchin  blastula,  mentioned 
above,  —  it  is  clear  that  the  factor  which  determines 
whether  one  individual  or  two  is  to  be  the  result  of 
the  process  of  development  lies  somehow  in  the  re- 
ciprocal relation  of  the  parts  of  the  blastula.  This 
may  be  illustrated  in  another  way.  If  the  zygote, 
after  the  first  cleavage,  be  placed  in  sea-water  which 
lacks  calcium,  the  two  cells  will  separate,  and,  if 
replaced  in  normal  sea-water,  each  will  develop  into 
a  perfect  individual.  As  in  the  previous  case,  two 
individuals  arise  from  one  zygote.  Here  it  is  clear 
that  the  absence  of  contact  with  the  one  blastomere 
determines  that  the  substance  of  the  other  shall 
differentiate  into  one  individual  instead  of  into  half 
a  one,  as  it  ordinarily  would  do.  The  result  is,  how- 
ever, always  the  specific  type. 

Heteromorphosis.  —  Although  the  regeneration  of 
a  new  appendage  or  other  part  in  nearly  every  case 
conforms  to  the  specific  type,   and  the  process  is 


190 


GENERAL  BIOLOGY 


essentially  one  of  "  restitution,"  yet  occasionally 
the  formative  forces  get  off  the  morphogenetic  track, 
and  a  wholly  abnormal  structure  results.     In  certain 

Crustacea,  such  as 
the  crayfish,  if  the 
experimenter  cuts 
off  one  of  the  eye- 
stalks,  another  will 
grow  to  replace  it, 
but  if,  in  addition 
to  severing  the  eye- 
stalk,  he  also  de- 
stroys the  deeper- 
lying  ganglion,  the 
resultant  growth 
is  not  an  eye-stalk 
at  all,  but  an 
antenna-like  structure.  (See  fig.  70.)  It  is  inter- 
esting to  find  that  the  new  organ  is  still  of  the  crus- 
tacean type.  Such  a  diversion  of  the  normal  path 
of  differentiation  is  termed  heteromorphosis. 


Fig.  70.  —  Regeneration  of  antenna-like 
organ  in  place  of  eye-stalk,  in  Palcemon. — 
(From  Morgan,  after  Herbst.) 


Theories  of  Morphogenesis.  —  The  nature  of  the 
changes  involved  in  the  ontogeny  of  both  plants 
and  animals  has  been  briefly  outlined.  The  ques- 
tion arises :  Why  does  the  germ  always  give  rise 
to  precisely  the  type  characteristic  of  the  species 
and  no  other  .^  Nothing  that  our  microscope  can 
tell  us  of  the  young  embryo  within  the  eggshell 
of  a  pigeon  or  a  sparrow  gives  us  the  slightest  clue 
as  to  why  the  culmination  of  the  development  of  one 


ONTOGENESIS  191 

is  different  from  tliat  of  the  other.  More  funda- 
mental is  the  question  :  Why  does  the  shapeless  germ 
take  form  at  all?  Nothing  that  we  can  learn  of 
its  nature  or  its  structure  gives  us  any  reason  for 
believing,  a  priori,  that  it  will  shape  itself  into  an 
individual  that  resembles  its  parents.  This  is  an 
old  problem,  and  its  solution  was  attempted  long 
before  our  modern  instruments  for  research  gave  us 
the  insight  into  developmental  processes  we  now 
possess. 

Preformation.  —  Suggested  perhaps  by  the  struc- 
tures found  within  the  flower-bud,  or  the  insect 
chrysalis,  the  idea  was  long  current  that  the  germ 
contains  within  itself  the  whole  organism  in  minia- 
ture and  that  development  consists,  simply,  in  an 
unfolding  and  enlarging  of  this  preformed  indi- 
vidual. The  chief  elaborator  of  this  speculation  was 
Bonnet  (1720-1793),  but  such  a  great  naturalist 
as  Cuvier  also  subscribed  to  the  doctrine.^  Since 
the  generation  "  A  "  was  preformed  in  the  genera- 
tion "  B,"  then  its  descendants  must  also  have  been 
there  preformed,  and  so  on.  The  germ-plasm, 
therefore,  was  conceived  of  as  a  sort  of  nest  of  Chinese 

^  There  was  a  division  of  opinion  as  to  whether  the  preformed  indi- 
vidual existed  in  the  egg  or  in  the  sperm.  Some  held  that  the  former  is 
the  case  and  that  the  function  of  the  sperm  is  merely  to  fructify  or  "fer- 
tilize" the  dormant  egg.  These  speculators  were  denominated  "ovists." 
On  the  other  hand  the  "  animalculists  "  contended  that  the  egg  is  merely 
dead  matter  serving  for  nourishment,  whereas  the  sperm  is  active  and 
"alive."-  Moreover,  with  the  primitive  microscopes  of  the  time,  they 
had  no  difficulty  in  distinguishing  head,  arms,  legs,  and  other  structures 
in  the  sperm. 


192  GENERAL  BIOLOGY 

boxes,  each  one  inclosing  a  series  of  increasingly 
smaller  ones.  Indeed  the  theory  was  called  the 
"  encasement  "    theory. 

Epigenesis.  —  Wolff,  the  father  of  modern  em- 
bryology, investigated  the  developing  chick  in  the 
shell,  discovering,  among  other  things,  that  the  heart 
actually  comes  into  existence  after  development 
begins,  and  was  forced  to  conclude  that  there  is  no 
evidence  whatever  of  the  preexistence  of  the  chick 
in  the  germ  of  the  egg.  In  his  Theoria  Generationis 
(1759)  he  advanced  the  hypothesis  of  epigenesis, 
according  to  which  the  development  of  the  germ 
involves  the  coming  into  existence  of  new  structures 
with  each  generation.  It  was  thus  in  direct  con- 
flict with  the  preformation  concept  held  by  the 
majority  of  eighteenth-century  naturalists  and 
philosophers.  As  for  the  means  of  this  epigenetic 
development,  he  conceived  of  a  specific  internal 
energy  or  force  (vis  essentialis)  that  permeates  living 
matter.  Development  (in  the  hen's  egg)  is  not 
brought  about  by  the  heat  of  incubation,  but  by 
the  operation  of  this  somewhat  mystic  internal 
force.  Wolff's  results  and  speculations  were  a  long 
time  in  gaining  acceptance,  but  the  gradual  im- 
provement in  microscopes,  and  in  technique, 
made  it  impossible  to  accept  the  naive  preforma- 
tion of  the  earlier  school,  and  the  biological 
world  became  persuaded  to  the  epigenetic  way  of 
thinking,  without,  however,  accepting  the  ''vis 
essentialis, '''' 


ONTOGENESIS  193 

Weismannism.  —  In  the  course  of  time  the  pen- 
dulum again  swung  back  toward  the  preformation 
standpoint.  The  increase  in  knowledge  of  the  data 
of  heredity,  and  a  more  exact  understanding  of  the 
cellular  phenomena  of  zygosis  and  ontogeny,  forced 
speculative  biologists  to  refer  back  the  structures 
that  develop  in  morphogenesis  to  some  sort  of  pre- 
existing structure  in  the  gametes.  Under  the  hand 
of  Weismann  (1834-  )  this  became  an  elaborate 
architecture  of  determining  particles  of  ultra-micro- 
scopic size,  each  of  which  is  the  causal  agent  in  bring- 
ing about  the  development  of  some  part  of  the  organ- 
ism. Weismann  laid  much  emphasis  on  the  concept 
of  the  continuity  of  the  germ-plasm,  in  contrast  to 
the  soma,  a  matter  already  discussed.  Development 
thus  becomes  a  mere  sorting  out  of  determinants, 
and  the  organism  is  a  sort  of  mosaic.  One  obvious 
corollary  of  such  an  hypothesis  is  the  fact  that 
nothing  that  may  befall  the  soma  after  development 
begins  can  have  any  influence  in  modifying  the 
result  of  development  in  a  succeeding  generation, 
since  each  generation  develops  in  strict  accordance 
with  the  determinants  in  the  germ-cells.  Most  ex- 
periments seem  to  prove  that  this  is  so.  On  the 
other  hand,  the  facts  of  regeneration  and  regulation, 
just  cited,  are  a  strong  argument  against  such  an 
inflexible  mosaic  development.  For  this  and  other 
reasons,  the  elaborate  and  complicated  architecture 
which  Weismann  postulated  to  exist  in  the  germ- 
cells  -is  not  considered  by  modern  biologists  really  to 
exist.     Nevertheless,  for  many  reasons,  particularly 


194  GENERAL  BIOLOGY 

on  account  of  the  discoveries  in  Mendelian  inher- 
itance/ a  great  many  biologists  believe  that  the 
development  of  the  structural  characteristics  of 
animal  and  plant  individuals  is  dependent  upon  the 
presence  of  "  something  "  in  the  germ-cell  to  which 
the  name  determiner  is  given.  This  determiner  is  a 
physical  entity  of  some  sort,  however,  and  very 
different  from  the  vis  essentialis  of  Wolff. 

Vitalism  and  Mechanism.  —  The  more  exact  be- 
comes our  knowledge  of  the  processes  of  differentia- 
tion and  development,  the  more  wonderful  appears 
the  delicate  adjustment  of  forces  that  brings  about 
the  final  result.  The  correspondence  of  time  and 
place  in  development  is  at  present  particularly 
difficult  to  comprehend.  Why,  for  instance,  does 
an  organ,  let  us  say  a  finger,  develop  at  precisely  the 
time  and  place  necessary  to  produce  a  symmetrical 
whole  ?  Why  does  an  organ  develop  in  apparent 
anticipation  of  a  subsequent  need.^  The  vitalists 
believe  that  no  known  laws  of  matter  can  account 
for  the  adaptation  of  means  to  the  end,  which  we 
are  constantly  confronted  with  in  the  study  of  mor- 
phogenesis, and  that  it  is  necessary  to  postulate  a 
non-mechanical  principle  or  "  vital  force,"  to  which 
various  names  are  given,  which  is  a  guiding  and  con- 
trolling agency  in  directing  the  course,  not  only  of 
development,  but  of  life  processes  in  general.  A 
sculptor  in  modeling  a  statue  must  have  a  pretty 
clear  idea  in  his  mind  of  just  what  he  expects  to 

1  See  next  chapter. 


ONTOGENESIS  195 

realize  in  the  completed  work:  he  works  toward  a 
definite  end,  and  his  preliminary  efforts  are  con- 
ditioned by  the  sort  of  final  result  he  wishes.  To 
many  observers  the  processes  of  development  seem 
equally  conditioned  upon  the  nature  of  the  final 
result,  and  it  is  hard  to  see  how  such  events  could 
come  to  pass  without  the  help  of  some  guiding 
agency,  like  the  sculptor  in  the  previous  comparison. 
Such  a  point  of  view  is  called  teleological.  Human 
action  is  so  constantly  purposive  that  the  untrained 
mind  unconsciously  reads  into  all  the  activities  of 
Nature  a  similar  purpose.  A  bygone  generation, 
but  by  no  means  an  unintellectual  one,  could  see 
no  way  of  accounting  for  the  movements  of  sun, 
moon,  and  planets  except  by  postulating  the  assist- 
ance of  angels  who  pulled  and  pushed  them  along 
their  appointed  courses.  With  the  increase  of 
knowledge  of  celestial  mechanics  it  became  clear 
that  the  intervention  of  the  imaginary  angels  is 
not  necessary,  and  the  explanation  of  the  move- 
ments of  heavenly  bodies  became  an  impersonal  or 
mechanical  one.  In  the  same  way,  to  "  explain  " 
the  complex  processes  of  development,  it  is  not 
necessary  to  call  upon  the  guiding  help  of  some 
hypothetical  vital  force,  even  though  our  knowl- 
edge of  developmental  mechanics  is  still  far  too 
inadequate  to  explain  the  observed  phenomena. 

One  of  the  most  prominent  students  of  the  rela- 
tions of  plants  to  their  surroundings  ^  says :  "  Each 
year    the    list    of    *  vitalistic    activities '    of    plants 

1  H.  C.  Cowles. 


196  GENERAL  BIOLOGY 

becomes  more  and  more  restricted  through  the  estab- 
lishment of  a  definite  physical  or  chemical  cause  for 
what  had  been  thought  to  have  a  vitalistic  explana- 
tion, while  never  in  the  history  of  science  has  any 
phenomenon,  once  explained  on  a  physical  or  chemi- 
cal basis,  later  been  found  to  be  vitalistic."  ^ 

Summary.  —  In  each  species  of  plant  or  animal 
there  is  a  continuous  and  unbroken  succession  of 
individuals  that  constantly  replace  one  another. 
There  is  no  authentic  instance  of  any  individual 
form  of  life  coming  into  existence  except  from  a 
preexisting  individual.  The  reproductive  process 
is  essentially  one  of  discontinuous  growth.  In  its 
simplest  expression  it  involves  the  cutting  in  two  of 
a  parent  organism  to  produce  two  "  daughter " 
individuals.  Instead  of  a  half  of  the  parent  organ- 
ism, the  source  of  the  i\pw  individual  may  be  a  por- 
tion of  the  parental  tissue  (bud)  or  a  single  cell 
(spore  or  gamete).  In  the  case  of  the  spore  the 
new  individual  arises  by  direct  growth  and  meta- 
morphosis, but  in  the  case  of  the  gamete  it  arises 
from  a  zygote,  which  is  the  result  of  the  partial  or 
complete  fusion  of  two  gametes.  In  any  event  the 
specific  form  of  the  new  individual  is  attained  by 
differentiation  from   a  relatively  generalized    to    a 

1  The  author  is  aware  that  the  above  paragraph  gives  a  very  incomplete 
presentation  of  the  vitalistic  standpoint,  particularly  of  that  of  the  so- 
called  Neo-Vitalists.  There  are  many  kinds  and  degrees  of  vitalism,  but 
to  go  into  the  subject  in  detail  is  quite  outside  the  compass  of  a  work  of 
this  sort.  The  interested  student  is  referred  to  the  works  of  Driesch, 
Bergson,  Reinke,  Lovejoy,  etc. 


ONTOGENESIS  197 

relatively  specialized  condition.  There  is  a  striking 
parallel  in  the  course  of  the  differentiating  process 
in  the  ontogenesis  of  all  species.  The  similarity 
of  the  steps  in  any  two  different  forms  is  in  direct 
ratio  to  the  closeness  of  their  relationship.  Sexual 
reproduction  (by  gametes)  is  apparently  an  inci- 
dental specialization  and  not  a  necessity  for  the 
accomplishment  of  the  reproduction  process,  since 
experiment  has  shown  that  the  egg  contains  within 
itself  all  the  required  potentialities  for  individual 
development.  It  is  probable  that  the  same  would 
be  also  true  of  the  sperm  except  that  specialization 
has  deprived  the  latter  of  the  necessary  food  supply 
to  serve  as  a  source  of  energy.  The  accomplishment 
of  the  specific  form  is  not  only  brought  about  by  the 
differentiation  of  a  specialized  cell,  normally  pro- 
duced by  another  individual  (ontogenesis),  but  also 
is  manifest  in  the  restoration  of  structures  in  the 
same  individual,  when  these  are  abnormally  altered 
or  destroyed  (restitution). 


CHAPTER  VII 
VARIATION  AND   HEREDITY 

Variation.  —  When  we  see  twins  that  resemble 
each  other  closely,  or  two  unrelated  individuals 
that  have  many  features  in  common,  our  attention 
is  at  once  attracted,  and  the  fact  that  the  phenomenon 
excites  our  interest  attests  its  comparative  rarity; 
in  other  words,  we  are  accustomed  to  the  fact  that 
individuals  do  not  resemble  one  another,  and  the 
occasional  exception  is  therefore  conspicuous.  If 
we  apply  exact  measurements  or  other  criteria  to 
any  sort  of  plant  or  animal,  or  to  a  structural  part, 
and  compare  with  similar  measurements  on  other 
individuals,  we  find  that  the  same  thing  holds  true, 
—  that  variation  is  a  universal  phenomenon,  and 
duplication  almost  non-existent. 

The  analysis  of  this  fact  of  universal  variation 
resolves  itself  into  a  comparison  of  structures,  — • 
the  components,  so  to  speak,  that  go  to  make  up 
the  individual.  Thus,  if  we  wished  to  compare  the 
individual  seeds  in  a  handful  of  beans,  we  should 
describe  their  size,  shape,  color,  texture,  etc.  These 
components  are  technically  called  "  characters." 
It  is  obvious  that  a  complex  individual  may  be 
resolved  into  a  large  number  of  such  characters, 
displaying  all   sorts   of   variations   when   compared 

198 


VARIATION  AND  HEREDITY  199 

with  other  individuals.  These  may  be  described 
and  catalogued,  but  words  alone  will  hardly  suffice 
to  discriminate  the  finer  shades  of  distinction  be- 
tween so  many  classes.  To  seek  order  in  such  a 
chaos,  some  sort  of  mathematical  basis  must  be 
devised. 

If  we  study  a  group  of  a  hundred  men  with  regard 
to  a  single  character,  such  as  statiire,  we  find,  of 
course,  that  all  the  individuals  fall  within  rather 
definite  limits,  from  the  shortest  man  to  the  tallest, 
and  we  tnight  classify  them  by  arranging  them  in  a 
row  in  the  order  of  height.  The  line  connecting  the 
tops  of  the  heads  of  such  a  row  of  men  should  be 
irregular  and  jagged  and  would  defy  analysis.  Sup- 
pose, however,  that  we  group  such  a  lot  of  men  in 
classes  corresponding  to  the  various  statures,  and 
place  a  representative  of  each  class  with  his  heels 
on  a  base-line.  Then,  grouping  all  of  a  class  together 
(see  fig.  71),  one  in  front  of  another,  we  would  find 
that  the  line  connecting  their  heads,  when  viewed 
from  above,  is  of  a  very  different  sort  compared  with 
the  former  one.  Briefly,  the  shortest  rows,  that  is, 
the  fewest  individuals,  would  be  found  in  the  shortest 
and  tallest  classes,  and  the  longest  rows  in  the  inter- 
mediate classes.  Viewed  from  above,  the  outline 
marked  by  their  heads  would  describe  a  fairly  regular 
curve,  reaching  its  highest  point  in  the  middle,  and 
curving  down  to  the  base-line  in  both  directions.  If 
a  thousand  individuals  instead  of  a  hundred  were 
thus  arranged,  the  line  would  be  more  even,  since 
individual  differences  would  tend  to  merge  in  the 


200 


GENERAL  BIOLOGY 


general  average.  If  one  were  to  fire  a  thousand 
rifle  shots  at  a  target  and  then  sort  out  the  results  and 
classify  them  with,  regard  to  the  accuracy  of  the  hits, 
it  would  be  found  that  the  most  accurate  and  the 
least  accurate  ones  are  fewest  in  numbers,  and  that 
the  greatest  number  of  hits  is  somewhere  in  between. 
If  we  plot  out  the  result  on  paper,  in  the  same  way 


Fig.  71. 


Bird's-eye  view  of  forty  men  arranged  in  files  by  classes  of 
stature.  —  (Davenport.) 


that  we  arranged  the  men  above  described,  a  similar 
curve  will  be  secured.  The  same  result  will  be 
obtained  from  any  large  array  of  data,  the  distribu- 
tion of  which  depends  upon  chance.  An  ingenious 
device  invented  by  Gal  ton  illustrates  this  mechan- 
ically. 

A  shallow  oblong  box  (fig.  72)  is  constructed,  one 


VARIATION  AND  HEREDITY 


201 


side  of  which  is  of  glass.  Toward  one  end  a  number 
of  longitudinal  compartments  are  formed  of  strips  of 
tin;  at  the  other  end,  a  sort  of 
funnel  is  constructed  in  the  same 
way.  Between  the  tw^o  is  a  field  of 
pins  inserted  alternately.  The  ap- 
paratus is  provided  with  a  handful 
of  shot  before  the  glass  cover  is 
put  on.  When  the  box  is  inverted, 
the  shot  all  run  back  into  the  com- 
partment behind  the  funnel. 


"Then,  when  the  box  is  tilted, 
the  shot  passes  through  the  funnel, 
and  issuing  from  its  narrow  end, 
scampers  deviously  down  through 
the  pins  in  a  curious  and  interest- 
ing way,  each  of  them  darting  a 
step  to  the  right  or  left  as  the  case 
may  be,  every  time  it  strikes  a 
pin.  The  pins  are  disposed  in  a 
quincunx  fashion,  so  that  every 
descending  shot  strikes  against  a 
pin  in  each  successive  row.  The 
cascade  issuing  from  the  funnel 
broadens  as  it  descends,  and  at 
length  every  shot  jfinds  itself  caught  in  a  compartment 
immediately  after  freeing  itself  from  the  last  row  of  pins." 


Fig.  72.  — Gal  ton's 
mechanical  device  for 
illustrating  the  law 
of  the  frequency  of 
error,  and  the  dis- 
tribution of  variates 
in  the  normal  curve. 


When  we  examine  the  disposition  of  the  shot  in  the 
compartments,  we  find  that  the  greatest  number 
is  to  be  found  in  the  middle  compartment  (if  the 
apparatus  be  held  vertically),  and  that  the  com- 
partments   on    either    side    contain    a    diminishing 


^£02 


GENERAL  BIOLOGY 


number.  In  other  words  the  Kne  that  connects  the 
tops  of  the  columns  describes  the  same  sort  of  a 
curve  ^  that  we  secure  when  we  plot  out  the  heights 
of  a  large  number  of  men. 

Nearly  all  the  obvious  variation  of  organisms  is 
of  the  kind  just  described.  Mathematical  analysis 
gives  no  clue  to  the  nature  of  the  individual  variate, 
and  for  this  reason  such  variation  is  frequently 
called  fortuitous,  i.e.  random  or  unpredictable.  Nev- 
ertheless the  mathematical  values  obtained  from  the 
analysis  of  a  mass  of  such  data  are  very  accurate 
and  certain. 


2600  LEAVES 

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— 

NORMAL  CURVt 

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NUMBER  OF  V-EINS  MEAN 

a  6 

Fig.  73.  —  The  Normal  Curve.     Veins  in  beech-leaves.  —  (From  Daven- 
port, after  Pearson.) 


Types  of  Variation  Curves.  —  The  theoretical  or 
normal  binomial  curve  is  perfectly  symmetrical 
(fig.  73).     The  classes  may  be  marked  off  along  the 

1  The  curve  is  that  known  in  mathematics  as  the  binomial  curve 
(the  expansion  of  the  expression  [p  +  g]"). 


VARIATION  AND  HEREDITY  203 

base-line  (a-b) ;  in  the  diagram  these  run  from  9 
to  23.  These  figures  happen  to  represent  the  vari- 
ation in  the  number  of  veins  in  beech  leaves,  but  they 
might  represent  the  limits  of  weights  of  seeds  in 
grams,  or  the  number  of  spines  in  a  fish's  fin,  or  any 
other  measurable  character.  The  number  of  variates 
in  each  class  is  represented  by  the  cross-lines,  each 


Fig.  74.  —  Two  symmetrical  curves  illustrating  the  value  of  "a-"  as  a 
measure  of  variability  (see  text). 

line  standing^  for  forty  individuals.  It  will  be  noted 
that  the  greatest  number  of  veins  falls  on  16,  which  is 
very  nearly  midway  between  9  and  23.  This  middle 
point  is  called  the  mode  or  mean.  Again,  there  is  a 
point  on  each  half -curve  where  the  curvature  changes 
from  concave  to  convex  (point  of  inflection).  Let 
us  now  compare  the  curves  in  fig.  74.  In  both  of 
these  curves  the  mode  is  the  same.  The  higher 
curve,  compared  with  the  lower,  shows  that  the 
variates  in  the  former  array  are  concentrated,  as  it 


204  GENERAL  BIOLOGY 

were,  about  the  median  dimension,  whereas  in  the 
flatter  curve  they  are  distributed  more  evenly  along 
the  whole  base-line.  In  other  words,  the  bulk  of 
the  individuals  of  the  first  lot  are  much  alike,  or, 
as  we  say,  are  not  so  variable  as  in  the  other  lot. 
It  is  evident  that  the  flatter  the  curve,  the  farther 
away  from  the  modal  axis  {M)  moves  the  point  of 
inflection.  The  measure  of  the  line  connecting  this 
point  and  the  modal  axis  (designated  by  the  Greek 
letter  o")  thus  becomes  the  measure  of  variability, 
being  greater  in  proportion  as  the  curve  is  flatter, 
i.e.  in  proportion  to  the  greater  variability. 

Asymmetrical  Variation.  —  In  actual  observations 
the  variation  curve  for  organisms  is  rarely  quite 
symmetrical ;  that  is,  the  mode  is  somewhat  nearer 
one  extreme  than  the  other.  The  index  of  varia- 
bility may  be  calculated  for  such  curves  as  for 
symmetrical  ones.  If  the  mode  is  much  nearer  one 
extreme  than  another,  the  curve  is  spoken  of  as 
"  skew."  Such  a  curve  frequently  indicates  that 
selection  has  taken  place  in  the  material  examined, 
and  the  variates  have  been  eliminated  dispropor- 
tionately. 

Discontinuous  Variation.  —  In  measuring  the  di- 
mensions of  an  organ  or  in  estimating  weights,  the 
number  of  classes  obtainable  is  limited  only  by  the 
accuracy  of  our  own  observations,  and  the  gradations 
of  variation  are  continuous,  as  is  indicated  by  the 
symmetrical  curves  which  we  usually  obtain.  Such 
variations  as  the  above  are  due  to  the  variability 


VARIATION  AND  HEREDITY  205 

in  the  material  or  substance  of  the  organs  and  are 
therefore  called  substantive  (Bateson).  On  the  other 
hand,  the  number  of  parts  may  vary,  as  with  flower- 
petals,  or  the  joints  of  an  appendage,  or  body- 
segments  (as  of  a  worm).  Such  variation  is  termed 
meristic.  It  is  clear  that  there  can  be  no  inter- 
mediate between  a  three-leaf  clover  and  a  four-leaf 
clover.     The  fourth  leaf  is  a  perfect  leaf,  no  matter 


Fig.  75.  —  Rhinoceros  beetles  (Xylotrupes  gideon) ;  I,  "high"  male; 
II,  "low"  male;  A,  the  cephalic  horn.  The  legs  are  omitted  for  the 
sake  of  clearness.  —  (After  Bateson.) 


how  small  it  may  be.  Meristic  variation  differs, 
then,  from  substantive  variation  in  being  discon- 
tinuous instead  of  continuous. 

Discontinuity  of  variation,  however,  may  be  dis- 
covered in  substantive  as  well  as  meristic  characters. 
When  we  plot  a  curve  of  measurements  for  a  char- 
acter, we  sometimes  find  that  it  apparently  shows  two 
modes,  or  is  "double-humped."  (See  figs.  75,  76.) 
Such  a  curve  is  impossible  to  analyze  by  the  usual 


206 


GENERAL  BIOLOGY 


methods,  for  it  shows  that  the  material  is  not  homo- 
geneous. In  reahty  we  have  two  groups,  with  two 
modes,  but  members  of  both  groups  are  mingled 
within  the  range  of  the  classes  that  are  common 
to  both,  and  in  consequence  we  cannot  determine 

the  limits  of  the 
curves  that  in- 
tersect. 

An  example  of 
this  is  figured  be- 
low. In  rhinoc- 
eros beetles  there 
are  two  long 
horns  which  pro- 
ject forward  from 
the  head  and  the 
thorax.  Bateson 
measured  the 
horn  on  the  heads 
of  some  342  bee- 
tles and  found 
that,  with  respect 
to  this  character,  the  insects  grouped  themselves 
in  two  classes,  the  curve  of  variation  being  a 
"  double-humped  "  one.  With  respect  to  the 
length  of  the  wing-covers,  however,  the  same  342 
beetles  were  shown  to  be  homogeneous,  the  curve 
for  this  measurement  having  but  one  mode. 
Again,  in  a  certain  Chrysanthemum,  De  Vries 
found  that,  although  the  number  of  ray-florets 
(often  miscalled   petals)  of   the  flower  varied   from 


Fig.  76.  —  Polygons  of  variation  based 
upon  the  measurements  of  342  rhinoceros 
beetles.  Two  modes  are  evident,  one  about 
4  mm.,  the  other  about  8|  mm. 


VARIATION  AND  .HEREDITY  207 

12  to  22,  yet  the  curve  of  variation  proved  itself  to 
be  a  double-humped  one  (see  cut),  with  one  mode  for 

13  florets  and  another  for  21.  De  Vries  rejected  all 
the  flowers  of  the  latter  class,  and  planted  the  seeds 
of  the  12-  and  13-rayed  flowers.  The  result  is 
indicated  in  the  third  curve,  "  C  " ;  all  the  flowers 
were  of  the  13-rayed  variety,  and  the  21-rayed  plants 
had  been  eliminated.  The  bearing  of  this  result 
will  be  discussed  further  on  in  another  connection. 


Fig.  77.  —  Discontinuous  variation  in  Chrysanthemum  segetum  (see  text). 

Mutations.  —  Discontinuous  variation  may  be 
qualitative  as  well  as  quantitative.  Although  such 
variations  are  conspicuous  and  striking  to  us,  they 
are  probably  of  the  same  sort  as  the  merely  quanti- 
tative or  meristic  variation.  This  kind  of  variation 
has  long  been  familiar  to  gardeners  and  horticul- 
turists under  the  name  of  "  sports."  A  classical 
example  studied  under  experimental  conditions  is 
the  evening  primrose  (CEnothera  lamarckiana) . 
This  species  is  supposed  to  have  been  introduced 
into  Europe  from  America,  although  it  is  no  longer 
found  wild  here.     Professor  De  Vries  secured  some 


208 


GENERAL  BIOLOGY 


wild  plants  from  a  field  near  Amsterdam  and  culti- 
vated the  stock  in  his  garden  for  eight  generations. 
He  began  with  nine  plants,  from  the  matured  seeds 
of  which,  two  years  later  (since  the  plant  is  a  bien- 
nial), he  grew  some  15,000  plants.  Ten  of  these 
were  of  a  different  appearance  from  the  rest,  and  these 
he  carefully  segregated.  Five  of  them  were  dwarf, 
with  small  leaves  but  full-sized  flowers,  and  these 
he  named  nanella;  the  other  five  had  broad  leaves 
and  a  luxuriant  growth,  and  the  flowers  were  all 
pistillate,  and  hence  could  not  produce  seed,  except 
when  crossed  with  another  variety;  this  form  he 
called  lata.  Both  nanella  and  lata  were  found  there- 
after in  every  culture  from  lamarckiana  seed,  until 
the  seventh  generation,  and  in  every  case,  when  self- 
fertilized,  the  former  "  bred  true."  In  the  third 
generation  a  new  form  appeared  which  he  called 
rubrinervis,  and  in  the  next,  three  others,  oblonga, 
albida,  and  gigas.     The  first  was  characterized  by 


Mutations  of  Oenothera  lamarckiana  (from  De  Vries) 


Gener- 
ation 

Gigas 

Alb  ID  A 

Ob- 

LONGA 

Rubri- 
nervis 

Lamarck- 
iana 

Na- 
nella 

Lata 

SCIN- 
TILLAN8 

I 

9 

II 

15,000 

5 

5 

III 

1 

10,000 

3 

3 

IV 

1 

15 

176 

8 

14,000 

60 

73 

1 

V 

25 

135 

20 

8,000 

49 

142 

6 

VI 

11 

29 

3 

1,800 

9 

5 

1 

VII 

0 

9 

3,000 

11 

5 

VIII 

5 

1 

1,700 

21 

1 

VARIATION  AND  HEREDITY  209 

reddish  veins  in  the  leaves,  oblonga  by  narrow  leaves, 
and  alhida  by  whitish  ones.  Gig  as  had  stems  nearly 
twice  as  thick  as  lamarckiana,  covered  with  dense 
foliage.  All  of  these  forms  reappeared  in  subsequent 
cultures  (compare  the  accompanying  table),  and  all 
bred  true  to  type. 

De  Vries  also  found  two  plants  of  lata  growing 
wild  in  the  field,  as  well  as  one  of  a  different  type 
with  smooth  leaves,  which  he  called  Icevifolia. 
This  form  not  only  bred  true  when  segregated,  but 
developed  other  types,  lata,  nanella,  rubrinervis,  and 
two  others  that  had  not  appeared  from  the  lamarck- 
iana seed. 

The  various  types  of  evening  primroses  described 
above  are  good  examples  of  discontinuous  variations 
which  have  been  watched,  as  it  were,  in  the  making. 
Such  variations  De  Vries  believes  to  be  very  different 
in  nature  from  ordinary  fluctuating  variations,  and 
to  them  he  gave  the  name  mutation. 

Correlated  Variation.  —  Since  all  the  parts  of  a 
normal  organism,  at  least  of  an  animal,  function  as  a 
unit,  this  harmony  of  action  demands  a  somewhat 
similar  harmony  of  structure.  We  should  expect 
to  find  that  organs  which  function  together  would 
vary  together.  The  analysis  of  comparative  measure- 
ments proves  this  to  be  a  fact.  The  abstract  index 
of  variation  of  one  organ  may  be  compared  with 
that  of  another  in  order  to  get  a  single  coefficient 
of  eorrelation  (usually  designated  "r").  The 
methods  for  securing  this  result  are  somewhat  com- 


210  GENERAL  BIOLOGY 

plicated  and  will  not  be  entered  upon  here.  The 
degree  of  correlation,  i.e.  the  value  of  "  r,"  is  usually 
expressed  in  fractions  of  unity;  in  other  words,  if 
r  =  1.0,  then  the  correlation  is  complete;  if  r  =  0.0, 
then  there  is  no  correlation.  A  correlation  of 
.25  or  less  is  usually  considered  too  low  to  be  signifi- 
cant, whereas  one  of  .75  or  more  is  very  high.  Fol- 
lowing are  some  values  of  r  for  various  dimensions 
of  man  and  the  flowers  of  the  Celandine  (Pearson). 

Stature  and  upper  leg  bone  .80    to  81 

Stature  and  upper  arm  bone  .77    to  81 

Stature  and  fore  arm  .37 

Stature  and  cephalic  index  .08 

Cephalic  index  and  intelligence  .029  to  .19 

Stamens  and  pistils  (Celandine)  .43    to  .75 

Stamens  and  sepals  (Celandine)  .06    to  .02 

Stature,  fathers  and  sons  .39 

Brothers  (various  characters)  .49 

There  is  every  reason  to  believe  that  similar 
correlations  exist  between  physiological  characters 
or  between  a  physiological  and  a  morphological 
character.  The  inherent  difficulties  of  detecting 
and  measuring  physiological  characters  limit  our 
knowledge  in  this  respect.  In  the  hormones  which 
are  manufactured  in  various  organs  and  transported 
to  other  parts  to  excite  physiological  response,  we 
can  picture  a  possible  mechanical  basis  for  such  cor- 
relations. It  is  obvious  that  they  may  constitute 
an  even  more  significant  factor  in  the  existence  of  the 
organism  than  morphological  correlations.  An  ex- 
ample  of  this   sort   of    correlation '  is    afforded    by 


VARIATION  AND  HEREDITY  211 

Professor  Pearson's  studies  upon  the  poppy,  in  which 
he  found  a  very  marked  correlation  between  the 
fertiHty  of  the  plant,  as  indicated  by  the  seeds 
developed,  and  the  number  of  stigmatic  bands  on 
the  seed  capsule. 

Effect  of  Life  Conditions  upon  Variation.  —  It  has 
been  shown,  both  by  comparative  studies  on  plants 
and  animals  at  different  ages,  and  also,  experi- 
mentally, by  varying  the  external  conditions,  that 
the  younger  stages  of  ontogeny  show  a  much  greater 
range  of  variability ;  in  other  words,  that  there  is  a 
progressive  reduction  of  variability  in  development. 
It  must  be  kept  in  mind  constantly  that  general 
statements  of  this  sort  apply  to  masses  of  individuals 
and  not  to  single  individuals.  The  weeding  out  of 
the  extreme  variates  in  the  course  of  development 
would  bring  about  a  similar  result. 

Suddenly  and  profoundly  altering  the  conditions 
of  life  appears  sometimes  to  increase  the  variability 
of  organisms.  The  English  sparrow  was  imported 
into  America  about  the  middle  of  the  last  century, 
and  its  new  surroundings  proved  so  favorable  that 
it  soon  spread  over  the  whole  country.  The  eggs 
of  868  sparrows  from  America  and  an  equal  number 
from  England  were  studied,  and  it  was  found  that 
the  former  were  considerably  more  variable  both  in 
size  and  in  color.  W^hether  one  is  justified  in  extend- 
ing such  a  conclusion  into  a  general  law  may  be 
questioned. 

Thus  it  has  been  long  thought  that  domestication 


212  GENERAL  BIOLOGY 

increases  the  variability  of  organisms.  Without 
doubt  a  greater  range  of  characters  is  permitted  to 
survive  by  the  florist  or  animal-breeder  than  would 
be  found  in  a  state  of  nature,  but  the  innate  capacity 
for  varying,  the  true  variability  of  the  species,  does 
not  appear  to  be  altered. 

In  some  way  not  understood  the  climate  of  a 
region  has  a  marked  influence  upon  the  variability 
of  the  plants  and  animals  inhabiting  such  a  region. 
A  botanist  has  compared  some  twenty-nine  kinds  of 
trees  grown  in  America  and  Europe  under  practically 
the  same  conditions.  His  results  are  quoted  by 
Darwin  as  follows :  "  In  the  American  species  he 
finds,  with  the  rarest  exceptions,  that  the  leaves  fall 
earlier  in  the  season,  and  assume  a  brighter  tint 
before  they  fall ;  that  they  are  less  deeply  toothed 
or  serrated ;  that  the  buds  are  smaller ;  that  the 
trees  are  more  diffuse  in  growth  and  have  fewer 
branchlets  ;  and  lastly,  that  the  seeds  are  smaller,  — 
all  in  comparison  with  the  corresponding  English 
species." 

Causes  of  Variation. —  Variations  may  be  of  two 
kinds :  those  that  are  induced  by  the  direct  action 
of  the  surroundings  and  those  that  arise  sponta- 
neously "  from  within  "  the  germinal  substance. 
The  former  are  called  somatogenic,  inasmuch  as  they 
affect  only  the  soma.  As  we  shall  see  a  little  farther 
along,  such  characters  affect  the  individual  without 
being  "  handed  on  "  to  its  posterity,  and  hence, 
from  the  standpoint  of  the  race,  are  transitory  in 


VARIATION  AND  HEREDITY  213 

nature.  On  the  other  hand,  variations  in  the  ger- 
minal substance,  in  the  nature  of  things,  will  affect 
future  generations.  Such  are  called,  therefore, 
germinal  or  blastogenic  variations. 

There  have  been  numerous  theories  dealing  with 
the  architecture  or  organization  of  the  germ-plasm, 
which  account  for  the  spontaneous  appearance  of 
new  characters,  but  all  such  hypotheses  are  founded 
upon  abstract  speculations  and  yield  no  verifiable 
explanation.  Nevertheless  the  knowledge  that  we 
have  gleaned  through  observation  and  experiment, 
of  what  goes  on  in  the  germ-cells  just  prior  to  the 
initiation  of  the  ontogenetic  process,  is  sufficient 
to  afford  us  a  good  clue  to  what  may  be  the  source, 
or  at  least  one  source,  of  germinal  variations. 

It  will  be  recalled  that  both  gametes  undergo  a 
process  of  maturation  just  before  zygosis  takes  place, 
in  the  course  of  which  the  amount  of  the  chromatin 
in  the  animal  egg  is  reduced  and  the  number  of  chro- 
mosomes in  both  gametes  is  halved.  This  phenom- 
enon is  known  as  reduction.  By  it  there  is 
frequently,  perhaps  always,  produced  two  kinds  of 
male  gametes  and  two  of  female  gametes.  The 
possibilities  of  fusion  between  these  two  sorts  of 
gametes  are  fourfold.  But  more  than  this;  in  the 
extremely  complicated  process  of  mitosis  it  is  very 
unlikely  that  the  chromosomes  should  in  every  way 
be  exactly  halved,  and  since  we  believe  the  nature 
of  the  organism  to  be  determined  in  large  measure 
by  the  chromatin  substance  which  is  passed  on  in 
the  germ-cells,  the  potentialities  of  the  zygote  may 


214  GENERAL  BIOLOGY 

be  highly  modified  by  very  sHght  changes  in  the 
germinal  substance  brought  about  by  such  unequal 
cleavage.  Furthermore,  it  is  believed  by  many 
that  there  is  an  individuality  of  the  chromosomes, 
such  that  it  is  not  a  matter  of  indifference  in  what- 
position  they  may  lie  in  the  conjugation  known  as 
synapsis,  which  occurs  just  before  reducing  division 
of  maturation.  It  has  been  shown  that  if  the 
chromosomes  are  all  qualitatively  different,  then 
the  following  relations  hold :  If  there  are  two 
chromosomes  in  the  somatic  cell  and  hence  but  one 
in  the  reduced  gametes,  there  are  two  possible  com- 
binations in  the  gametes  and  four  in  the  zygotes ; 
if  there  are  eight  in  the  somatic  cells,  there  will  be 
16  possible  combinations  in  the  gametes  and  ^56 
in  the  zygotes ;  if  16  in  the  somatic  cells,  then  there 
will  be  65,536  possible  combinations  in  the  zygote ; 
if  32  in  the  somatic  cells,  there  may  be  over  4,294,- 
467,296  possibilities  in  the  zygotes.  It  would  seem, 
therefore,  that  on  a  basis  of  chance  alone  abundant 
opportunity  for  blastogenic  variation  is  afforded  by 
the  mechanism  by  which  the  reduction  and  subse- 
quent recombination  of  chromosomes  is  brought 
about  in  maturation  and  zygosis. 

Heredity 

Heredity  and  Inheritance.  —  All  living  creatures, 
as  we  have  seen,  are  descended  from  ancestors 
which  they  resemble  more  or  less  closely.  The  facts 
of  variation  teach  us  that  the  resemblance  is  never 
an  actual  duplication,  yet  all  organisms  ''  conform  to 


VARIATION  AND  HEREDITY  215 

type,"  and  in  most  cases,  except  in  those  in  which 
a  marked  alternation  of  generations  has  been  de- 
veloped, the  resemblance  is  closest  between  the 
progeny  and  its  immediate  ancestors.  This  fact 
of  resemblance  between  relatives  is  called  Heredity. 
It  has  been  defined  as  "  the  genetic  relation  between 
successive  generations."  In  popular  writing  we 
often  read  of  "  The  Law  of  Heredity."  In  the  sense 
of  an  external  constraining  influence  that  creates 
or  compels  such  a  resemblance,  there  is  no  such 
thing,  of  course.  Laws  of  that  sort  do  not  exist 
in  the  world  of  Nature.  As  a  matter  of  fact.  He- 
redity implies  merely  a  comparison  between  related 
organisms  and  is  not  a  "  thing  in  itself." 

It  will  be  recalled  at  once  that  such  resemblance 
may  be  specific ;  that  is,  a  particularly  long  nose  or  a 
certain  cast  of  feature  may  be  the  common  posses- 
sion of  both  father  and  son.  Such  physical  traits, 
by  an  easy-going  analogy,  are  naturally  classed  in 
the  same  category  with  material  heritage  such  as 
property  and  lands,  which  descend  from  father  to 
son.  It  must  be  kept  in  mind,  however,  that  this 
is  an  analogy  only.  The  physical  features  of  the 
individual  are  the  result  of  development  from  a  rela- 
tively undifferentiated  germ-cell.  They  were  non- 
existent in  the  zygote.  On  the  other  hand,  these 
characteristics,  as  we  shall  see  later,  may  be  passed 
on  without  even  appearing  in  visible  form,  to  re- 
appear in  a  subsequent  generation.  For  this  and 
many  other  reasons  it  is  difficult  not  to  believe  that 
the  characteristics  of  the  individual  inhere  in  some 


216  GENERAL  BIOLOGY 

way  in  the  physical  make-up  of  the  germinal  sub- 
stance. Whatever  this  substance  may  be,  it,  with 
its  accompanying  potentialities,  is  spoken  of  as  the 
Inheritance  of  the  individual.  The  term  Heredity 
is  applied  to  the  larger  phenomenon,  by  virtue  of 
which  such  genetic  resemblances  universally  occur 
among  organisms.  Inheritance  is  concrete,  heredity 
is  abstract. 

Individual  Heredity  and  Racial  Heredity.  —  All 

the  individuals  of  any  given  sort  of  animal  or  plant 
resemble  one  another  more  or  less  closely,  else  we 
would  not  be  able  to  group  them  together.  For 
instance,  all  white  oaks  look  sufficiently  alike  so 
that  we  may  distinguish  them  easily  from  other 
oaks.  In  the  same  way,  the  physical  characteristics 
of  a  Chinese  or  an  Englishman  are  sufficiently  pro- 
nounced so  that  either  race  is  easily  recognizable ; 
yet  the  similarities  that  class  them  both  as  human 
beings,  on  the  one  hand,  and  the  peculiarities  that 
distinguish  them  as  individuals,  on  the  other  hand, 
are  equally  evident.  There  are  grades  or  degrees 
of  physical  resemblance  which  correspond,  in  general, 
with  the  closeness  or  remoteness  of  relationship  of 
individuals  or  groups. 

The  reason  that  this  is  so,  is  that  the  characteristics 
of  an  individual  are  not  alone  its  inheritance  from 
two  parents,  but  also  from  a  great  number  of  grand- 
parents and  other  ancestors  many  generations  back. 
It  is  obvious  that  there  must  be  much  mixing  and 
intercrossing  in  the  remote  parentage  of  any  indi- 


VARIATION  AND  HEREDITY  (217 

vidual.  Thus  there  is  a  racial  inheritance,  which 
is  the  common  possesssion  of  all  the  individuals  of 
any  group  or  race.  When  we  try  to  picture  the 
characteristic  features  of  this  sort  of  heritage,  we 
find  that  we  must  abstract  the  qualities  of  a  great 
number  of  individuals  and  make  a  sort  of  composite 
or  average  of  them,  i.e.  such  a  description  is  that 
of  an  ideal  individual  instead  of  a  real  one.  We 
unconsciously  do  this  whenever  we  form  a  mental 
picture  of  an  organism,  such  as  a  trout,  or  an  apple 
tree,  or  a  butterfly. 

Galton's  Law  of  Ancestral  Inheritance.  —  We  have 
found  that  one  fruitful  way  of  comparing  the  like- 
ness or  difference  between  two  groups  of  individuals 
is  by  calculating  the  abstract  index  of  correlation 
of  the  variations.  Correlating  the  resemblances  be- 
tween parents  and  offspring  gives  us  an  index  of  the 
degree  of  inheritance  which  we  may  assume  the  latter 
derive  from  the  former  (on  the  average,  not  indi- 
vidually). Sir  Francis  Galton,  a  famous  student 
of  heredity,  calculated  in  this  way  the  degree  of 
inheritance  received  by  the  offspring  from  parents 
and  from  more  remote  ancestors,  and  concluded 
that,  on  the  average,  the  individual  receives  one  half 
his  heritage  from  his  two  parents,  one  fourth  from 
his  four  grandparents,  one  eighth  from  his  great- 
grandparents,  one  sixteenth  from  the  great-great- 
granjdparents,  and  so  on.  Subsequent  calculations 
have  modified  these  fractions,  increasing  the  per- 
centage derived  from  the  immediate  parents   and 


ns  GENERAL  BIOLOGY 

decreasing  that  from  more  remote  ancestors.  But 
it  seems  to  remain  true  that  half  or  more  than  half 
of  the  individual's  inheritance  is  derived  from  his 
immediate  parents. 

The  sort  of  characteristics  with  which  Galton 
worked  were  such  as  vary  continuously  ;  for  example, 
stature  and  other  measurable  qualities.  It  is  a 
frequent  characteristic  of  such  structures,  that  they 
blend  or  mix  in  inheritance.  The  children  of  tall 
fathers  and  short  mothers  tend,  on  the  average,  to 
be  neither  tall  nor  short,  but  rather  intermediate 
in  height.  It  has  bepn  found  that  this  is  not  at 
all  true  of  a  great  many  kinds  of  inherited  characters. 
The  eye-color  of  a  child  whose  mother  has  blue  eyes 
and  his  father  brown  eyes  will  not  be  a  blend  or  a 
mixture  of  the  two  colors,  but  will  be  like  either 
one  parent  or  the  other.  (There  is  an  occasional 
exception.)  Again  the  cross  between  a  white  flower 
and  a  red  flower  will  frequently  be  striped  red  and 
white.  In  such  cases,  it  is  evident  that  Galton's 
law  of  ancestral  inheritance  does  not  apply.  It  is 
customary  to  call  the  last  type  particulate  inheri- 
tance, the  second  type  (such  as  eye-color)  alternative 
inheritance,  and  the  first  type,  blended  inheritance. 
Whether  or  not  there  is  any  fundamental  difference 
between  the  three  types  has  not  been  fully  established. 
Recent  breeding  experiments  with  both  plants  and 
animals  have  shown  that  an  unexpectedly  large 
proportion  of  inheritance  is  of  the  alternative  type, 
and  a  correspondingly  small  proportion  truly  blended 
or  particulate. 


VARIATION  AND  HEREDITY 


219 


Filial  Regression.  —  Galton  also  worked  out,  by 
statistical  methods,  a  law  which  he  named  the  law 
of  filial  regression.  If  we  take  the  mean  height 
of  a  group  of  fathers,  selected  because  they  are 
more  than  normally  tall,  and  compare  it  with  the 
mean  height  of  their  sons,  we  shall  find  that  the 

Inches 


Fig.  78.  —  Scheme  to  illustrate    Galton's  Law  of   Regression.  —  (From 

Walter.) 

latter  average  nearer  mediocrity ;  that  is,  they  "  re- 
gress "  toward  the  normal.  Regression  does  not 
mean  degeneration,  but  merely  a  shifting  toward  the 
mass-center  of  the  type.  It  may  be  upward  as  well 
as  downward,  i.e.  the  sons  of  short  fathers,  on  the 
average,  tend  to  be  of  somewhat  greater  stature, 
again  a  shift  toward  the  mean  of  the  type. 


220  GENERAL  BIOLOGY 

Effect  of  Selection  in  Heredity.  —  The  fluctuating 
variations  which  group  themselves  in  a  probability 
curve  have  been  especially  noticed  in  domesticated 
plants  and  animals,  although  they  are  probably 
no  more  or  less  numerous  in  the  wild  types.  In  the 
former,  man  has  been  long  accustomed  to  pick  out 
or  "  select  "  the  particular  type  that  he  favored  for 
breeding  purpose,  knowing  that  the  progeny  will  be 
similar.  Thus,  it  is  desirable  in  the  culture  of 
sugar-beets  to  secure  as  high  a  percentage  of 
sugar  as  possible.  The  amount  of  sugar  which  the 
beet-root  stores  up,  although  modified  by  external 
conditions,  such  as  sunshine  or  its  lack,  is  due  in 
great  part  to  the  inherent  qualities  of  the  seed, 
that  is,  is  an  hereditary  character.  In  unselected 
stock  the  percentage  of  sugar  is  usually  7  per  cent  to 
14  per  cent.  If  only  those  beets  with  the  highest 
percentage  of  sugar  be  taken  for  seed,  it  is  possible 
to  increase  this  average  in  two  to  three  years  (that 
is,  to  shift  the  mode  of  the  curve  toward  the  maximum 
extreme),  until  a  percentage  of  about  20  per  cent  is 
attained.  But,  curiously  enough,  further  selection 
will  not  increase  that  percentage,  and  moreover 
constant  selection  is  necessary  in  order  that  this 
maximum  be  maintained.  Such  an  experiment 
shows,  as  did  Galton's  calculations,  that  the  bulk 
of  inheritance  is  derived  from  the  immediate  parents 
(otherwise  selection  would  not  alter  the  mode  so 
rapidly),  and,  secondly,  that  there  is  a  definite  limit 
beyond  which  selection  cannot  alter  the  degree  of 
inheritance,  or  perhaps,  we  may  say,  beyond  which 


VARIATION  AND  HEREDITY 


221 


the    preponder-  Pn^E.  Line 

ating  influence 
of  the  immedi- 
ate parents  can- 
not overcome 
the  cumulative 
weight  of  the 
mediocrity  of 
the  more  remote 
ancestry. 

Pure  Lines  in 
Heredity. — Ap- 
proaching the 
problem  from  a 
different  angle, 
a  Danish  biolo- 
gist, Johanssen, 
experimented 
by  measuring 
the  effect  of  se- 
lection upon  a 
form  in  which 
cross-fertiliza- 
tion may  be 
eliminated.  The 
common  garden 
bean  is  adapted 
for  such  an  ex- 
periment, since  it  is  normally  self -fertilized.  He  ac- 
cordingly measured  the  breadth,  length,  and  weight 


Population 


Fig.  79.  —  Diagrams  showinfi. 
and  a  population  formed  by  their  union.  The 
beans  of  each  pure  line  are  represented  as  assorted 
into  inverted  test  tubes  making  a  curve  of 
fluctuating  variability.  Test  tubes  containing 
beans  of  the  same  weight  are  placed  in  the  same 
vertical  row.  —  (From  Walter,  after  Johanssen.) 


pare  lines 


222  GENERAL  BIOLOGY 

of  a  quantity  of  beans  and  plotted  the  variation 
curve  for  the  lot.  He  found  that  it  followed  the 
normal  curve,  i.e.  the  variations  were  of  the  fluctuat- 
ing sort,  grouping  themselves  about  a  mean.  He 
then  selected  beans  from  both  extremes  for  planting, 
and  saw  to  it  that  the  flowers  on  the  resultant  plants 
were  self -fertilized.  As  might  be  expected,  the 
progeny  "  took  after  "  the  parents ;  the  beans  of 
the  second  generation  that  were  descended  from  the 
lightest  beans  were  all  of  less  weight,  whereas  those 
from  the  heavier  were  all  heavier  than  the  average. 
The  variation  curves  for  the  progeny  of  each  bean 
were  also  of  the  normal  type,  but  with  a  much 
narrower  class  range  than  the  curve  for  a  miscel- 
laneous lot  of  beans  taken  at  random.  From 
these  smaller  groups,  each  the  progeny  of  a  single 
seed,  extreme  variates  were  selected  and  planted. 
But  the  progeny  of  these  seeds,  when  weighed,  were 
found  to  group  themselves  in  a  curve  with  essentially 
the  same  mode  as  that  of  their  progenitors.  That 
is,  within  the  group  of  plants  descended  from  a 
common  (self-fertilized)  ancestor,  selection  has  no 
influence,  one  way  or  the  other,  in  shifting  the  mode 
for  the  next  generation.  Johanssen  called  these 
smaller  groups  within  the  mass  of  the  species  pure 
lines,  or  genotypes,  in  contrast  to  the  larger  group 
which  all  together  they  go  to  make  up,  and  which 
he  called  the  phsenotype.  Ordinarily,  cross-fertili- 
zation is  continually  mixing  one  pure  line  with 
another,  and  as  the  greater  number  of  such  pure  lines 
group   themselves   about  the  mean    of    the    whole 


VARIATION  AND  HEREDITY  223 

mass,  the  curve  for  any  random  lot  of  individuals  is  a 
normal  curve.  Artificial  selection,  however,  picks 
out  not  only  extreme  individuals,  but,  at  the  same 
time,  extreme  "  pure  lines."  Since  the  latter  are, 
themselves,  unmodified  by  selection,  a  rational 
explanation  is  at  once  offered  to  account  for  the 
puzzling  fact  that  a  race,  such  as  the  sugar  beet, 
may  respond  promptly  to  selection,  but  refuse  to 
respond  at  all  beyond  a  certain  point. 

Similar  results  have  been  obtained  for  protozoa 
(Paramecium),  in  which  the  habit  of  asexual  re- 
production enables  the  experimenter  to  isolate  his 
pure  lines  and  do  away  with  the  disturbing  effects 
of  crossing.  A  mutation  or  discontinuous  variation 
might  be  explained,  perhaps,  as  a  coming  into 
existence  of  a  new  "  pure  line,"  though  by  what 
method,  or  from  what  cause,  we  at  present  cannot 
say. 

Unit  Characters  and  Mendelian  Inheritance.  — 
The  statistical  study  of  inheritance  deals  only  with 
organisms  in  the  mass,  and  its  conclusions  are  in- 
applicable in  individual  instances.  The  more  pre- 
cise its  results,  the  more  abstract  they  become.  But 
heredity  is,  after  all,  a  personal  matter,  and  it  is 
of  the  greatest  interest  to  discover,  if  possible, 
just  what  are  the  laws  that  determine  the  distri- 
bution of  inherited  qualities  in  individual  instances. 
Within  recent  years  very  remarkable  progress  has 
been  made  in  the  solution  of  this  problem,  although, 
as  is  so  frequently  the  case,  the  increase  in  knowledge 


224  GENERAL  BIOLOGY 

has  chiefly  served  to  reveal  the  immensity  of  the 
fields  that  yet  remain  to  be  explored. 

The  beginning  of  this  line  of  work  we  owe  to  the 
efforts  of  a  monk  named  Mendel,  who  worked  in 
the  cloister  garden  of  his  monastery  at  Brunn 
(Austria),  in  the  middle  of  the  last  century.  The 
results  of  his  experiments  were  published  shortly 
after  the  publication  of  Darwin's  famous  "  Origin  of 
Species,"  and  were  so  completely  eclipsed  by  the 
controversies  arising  out  of  that  famous  hypothesis, 
that  they  were  buried  in  obscurity,  until  the  facts 
were  rediscovered  at  the  beginning  of  the  present 
century. 

The  material  that  Mendel  worked  with  was  the 
common  garden  pea,  which  affords  a  number  of 
diverse  and  easily  recognized  characters,  such  as 
the  habit  of  the  plant,  the  color  of  the  flower  and  of 
the  seed,  the  nature  of  the  seed-coat,  etc.  When 
Mendel  crossed  (hybridized)  peas  that  differed  with 
respect  to  these  characters,  he  found  that  such 
characters  behaved  as  independent  units.  Thus, 
in  his  first  and  classic  experiment,  two  strains  of 
peas  were  crossed,  a  tall  and  a  dwarf.  The  matured 
seeds  of  this  cross  were  saved  and  planted.  The 
plants  that  grew  from  them  were  all  tall.  It  made 
no  difference  whether  pollen  of  the  dwarf  were  used 
on  the  stigma  of  the  tall,  or  vice  versa,  the  result 
was  always  the  apparent  extinction  of  the  dwarf 
characters,  —  the  plants  were  all  tall.  When,  how- 
ever, the  flowers  of  these  tall  (hybrid)  peas  were  self- 
fertilized  and  the  matured  seeds  planted,  the  resulting 


VARIATION  AND  HEREDITY  225 

plants  were  of  two  classes.  Some  of  them  were  tall 
and  some  dwarf,  like  the  original,  in  the  approximate 
proportion  of  three  of  the  former  to  one  of  the  latter. 
Again  self-fertilizing  the  flowers  on  the  various  plants, 
he  found  that  the  seeds  from  the  dwarf  plants 
developed  dwarfs,  and  this  was  repeated  in  subse- 
quent generations,  whereas  some  of  the  tall  plants 
developed  only  tails  {i.e.  "  bred  true  "),  and  others 

TALL(Pure)  X  DWARF 


,TALL  ^. 
(Impure} 


TALL  DWARIi. 

,(787r        Ratio  3:1  (ZTT) 


I 


PURE  TALL  IMPURE  TALL  hwarr c 

(28)  Givina  Dwarfs  DWARF F^ 

Ratio  1:2      VZ) 

Fig.  80.  —  Diagram  of  the  ratios  obtained  by  Mendel  in  crossing  tall  peas 
with  dwarf.  —  (Punnett.) 

developed  both  tails  and  dwarfs,  again  in  the  ratio  of 
three  to  one. 

Two  important  facts  are  brought  out  in  this  ex- 
periment. First,  the  characters  "  tallness  "  and 
"  dwarf ness,"  whatever  their  predetermining  cause  ^ 
may  be,  exist  in  the  gametes  as  segregate  entities, 
sorting  out  in  mathematical  ratios  in  each  generation. 
To   such,    the   name   "unit   characters  "    has   been 

^  "  Tallness  "  in  peas  differs  from  tallness  in  human  beings  in  that  it 
depends  upon  the  characteristic  length  of  intemode  between  the  joints 
where  tKe  leaves  are  given  off,  i.e.  the  dwarf  pea  is  not  a  miniature  pea 
but  a  different  kind  of  a  pea  with  regard  to  this  one  character. 
Q 


226  GENERAL  BIOLOGY 

given.  Secondly,  these  two  "  units  characters  " 
bear  some  relation  to  each  other,  such  that  one  may 
be  suppressed,  as  it  were,  and  fail  to  develop  in  a 
given  generation,  and,  at  the  same  time,  be  passed 
on  to  another  generation,  there  to  become  manifest. 
Characters  that  are  thus  linked  together  in  couples 
have  been  called  by  recent  investigators  "  allelo- 
morphs." Tallness  and  dwarf ness  in  peas  are  there- 
fore hereditary  qualities  that  fall  in  the  category 
we  have  already  called  alternative  inheritance. 
Since,  in  the  presence  of  the  former,  the  latter  is 
unable  to  manifest  itself,  Mendel  used  the  adjective 
dominant  for  the  tall  character,  and  recessive  for  the 
dwarf.  A  diagram  will  serve  to  make  this  clearer. 
T  indicates  the  tall  character,  Z),  the  dwarf  character, 
the  parenthesis  denoting  the  recessive  or  latent 
condition  of  the  character.  The  first  hybrid  genera- 
tion is  called  the  filial  generation,  F,  the  second,  r2, 
and  so  on. 

In  considering  the  r2  generation,  in  which  the 
dwarf  character  reappears,  it  is  also  evident  that 
there  are  two  classes  of  tails.  For  subsequent 
breeding  (always  with  self-fertilization)  shows  that 
whereas  one  third  of  the  tails,  like  the  dwarfs,  always 
breed  true,  the  other  two  thirds  (one  half  ofthe  whole 
number)  contains  the  dwarf  character  in  a  latent 
or  recessive  form,  since  the  latter  reappear  again 
in  the  same  ratio,  3  : 1,  as  that  of  the  previous  genera- 
tion. 

If  we  assume  that  the  gametes,  whose  fusion  pro- 
duces the  zygote  and  hence  determines  the  characters 


VARIATION  AND  HEREDITY  227 

of  the  individual,  are  always  "  pure  "  for  one  or  the 
other  of  such  a  pair  of  alternative  characters,  then  the 
dwarfs  in  the  F3  generation  may  arise  from  the  fusion 
of  two  gametes,  both  of  which  contain  the  "  factor  " 
for  dwarfness.  They  breed  true  because  the  factor 
for  tall  is  absent.  We  are  accustomed  to  speak  of 
such  an  individual  as  homozygous  for  such  a  character. 
The  same  could  be  said  of  the  tails  that  breed  true. 

All  of  the  gametes 

T   X   D ^P 

T(D) F, 


produced  by  such 
a  homozygous  in- 
dividual would  be 

of    one    and     the      ^  ZJZ.  ZJZ.  1 

T  T(D)  T(D)  D- 

J I 


I     ^^ 

T    T  T(D)  T(D)  D  TT(D)  T(D)  D     D-'-Fg 


T    r 


same  kind.  On 
the  other  hand,  if 
a  gamete  that  car- 
ries the  factor  for     T  D — F4. 

tallneSS  fuses  with  Fig.  81.  —  Diagram  of  the  results  of  cross- 

,  1      .  •  ing  tall  and  dwarf  peas  ;    see  text.  —  (From 

one    that    carries    Punnett.) 
the  factor  for 

dwarfness,  the  latter  would  become  recessive  and 
the  indiviual  would  be  tall,  but  the  gametes  developed 
by  such  an  individual  might  be  of  the  two  kinds, 
one  carrying  the  factor  for  the  tall,  the  other  for 
dwarf.  An  individual  of  this  sort  is  called  heterozy- 
gous for  such  a  character.  Remembering  that  the 
purity  of  the  gamete  with  respect  to  these  characters 
is  postulated,  and  that  each  kind  is  produced  pre- 
sumably in  equal  numbers,  if  fertilization  occurs  at 
random,  then  by  the  law  of  chances  there  would 
be  twice  the  opportunity  for  heterozygous  as  for 


228 


GENERAL  BIOLOGY 


Fig.  82.  —  Mendelian  inheritance  in  nettles :  I,  the  leaves  of  the  two 
parents,  Urtica  pilulifera  (at  the  left)  and  Urtica  dodartii  (at  the  right)  ; 
the  serrated  type  of  leaf  is  dominant ;  //,  the  leaves  of  the  hybrid's 
offspring  (F2  generation)  ;  the  pure  gametes  are  represented  by  the  black 
and  white  disks,  and  the  "impure"  by  the  cross-barred  disks  ;  in  III,  the 
F3  generation,  their  descendants  are  constant  (right  and  left),  the  other 
two  fourths  sorting  out  as  before.  —  (From  Hertwig,  after  Strassburger.) 


VARIATION  AND  HEREDITY 


229 


TaU 
Parent 


gametes , 


\ 


o 

(0 

B 

•a 
I  O- 


€ 

-3 


F, 


o 


02 


homozygous  fusions  to  occur.     A  glance  at  the  dia- 
gram (fig.  83)  will  make  this  clear. 

One  fourth  would  be  pure  recessive  (DD),  two 
fourths  impure  dominant  ((D)  T)  and  (T  (D)), 
and  one  fourth  pure 
dominant  (TT). 
Now  T  (D)  and  (D) 
T  are  identical  and 
contain  the  recessive 
(D),  which  in  a  sub- 
sequent generation 
sorts  out  in  a  simi- 
lar fashion,  giving 
homozygous  dwarfs 
and  tails,  and  hetero- 
zygous tails.  The 
last  two  classes 
would  be  indistin- 
guishable on  account 
of  the  dominance  of 
the  tall  character, 
but  would  sort  out 
very  differently  in 
continued  breeding, 
as  experiment  shows. 
In  Mendel's  original  experiment  he  got  787  dwarfs 
and  277  tails  in  the  first  filial  generation,  and  of  the 
latter  28  were  pure  in  the  succeeding  generations. 
In  other  words,  the  actual  experimental  result  bore 
out  strongly  the  hypothesis  formulated,  which  is 
based   upon  the  assumption  of  the  purity   of  the 


^feneration 

Fig.  83.  —  Scheme  of  the  possible 
zygoses  of  the  gametes  of  two  parents  that 
diflfer  with  respect  to  one  character,  such  as 
the  tallness  and  dwarfness  of  Mendel's 
peas.  The  two  parents  breed  true,  that 
is,  have  but  one  kind  of  gamete  (homozy- 
gous). There  are  four  possible  combina- 
tions and  hence  four  theoretical  types  of 
zygotes.  Owing  to  the  phenomenon  of 
dominance,  three  of  these  cannot  be  out- 
wardly distinguished  from  one  another. 
—  (Punnett.) 


230  GENERAL   BIOLOGY 

gametes  on  the  one  hand,  and,  on  the  other,  the 
segregation  of  the  unit  characters  in  the  zygote  and 
their  chance  union  in  subsequent  zygoses.  Subse- 
quent investigation  has  shown  that  the  problem  is 
by  no  means  so  simple  as  might  be  inferred  from  the 
behavior  of  Mendel's  peas.  The  "  dominance " 
of  one  unit-character  over  another  has  been  shown 
to  be  imperfect  or  non-existent  in  many  cases,  and, 
more  striking  still,  the  character  of  the  first  hybrid 
generation  is  not  infrequently  quite  different  from 
either  of  the  immediate  parents.  A  remarkable 
instance  has  been  worked  out  in  some  breeds  of 
the  domestic  fowl.  The  following  quotation  from 
Punnett's  "  Mendelism  "  gives  the  essential  facts : 
"  Many  of  the  different  breeds  of  poultry  are 
characterized  by  a  particular  form  of  comb,  and  in 
certain  cases  the  inheritance  of  these  has  been  care- 
fully worked  out.  It  was  shown  that  the  rose  comb 
(fig.  84,  B),  with  its  flattened  papillated  upper  sur- 
face and  backwardly  projecting  pike,  was  dominant 
in  the  ordinary  way  to  the  deeply  serrated  high 
single  comb  (fig.  84,  C)  which  is  characteristic  of 
the  Mediterranean  races.  Experiment  also  showed 
that  the  pea  comb  (fig.  84,  A),  a  form  with  a  low 
central  and  two  well-developed  lateral  ridges,  such  as 
is  found  in  Indian  game,  behaves  as  a  simple  domi- 
nant to  the  single  comb.  The  interesting  question 
arose  as  to  what  would  happen  when  the  rose  and 
the  pea,  two  forms  each  dominant  to  the  same  third 
form,  were  mated  together.  It  seemed  reasonable 
to  suppose  that  things  which  were  alternative  to 


VARIATION  AND  HEREDITY 


231 


the  same  thing  would  be  alternative  to  one  another  — 
that  either  rose  or  pea  would  dominate  in  the  hybrids, 
and  that  the  F2  generation  would  consist  of  domi- 
nants and  recessi ves  in  the  ratio  3:1.  The  result 
of  the  experiment  was,  however,  very  different.     The 


Fig.  84.  —  Fowl's  combs:  A,  pea;  B,  rose;  C,  single;  D,  walnut. 

cross  rose  x  pea  led  to  the  production  of  a  comb 
quite  unlike  either  of  them.  This,  the  so-called 
walnut  comb  (fig.  84,  Z)),  from  the  resemblance  to 
the  half  of  a  walnut,  is  a  type  of  comb  which  is 
normally  characteristic  of  the  Malay  fowl.  More- 
over,  when   these  Fi  birds  were  bred  together,   a 


232  GENERAL  BIOLOGY 

further  unlooked-for  result  was  obtained.  As  was 
expected,  there  appeared  in  the  F2  generation  the 
three  forms,  walnut,  rose,  and  pea.  But  there  also 
appeared  a  definite  proportion  of  single-combed 
birds,  and  among  many  hundreds  of  chickens  bred 
in  this  way  the  proportions  in  which  the  four  forms, 
walnut,  rose,  pea,  and  single,  appeared  was  9:3:3:1. 
Now  this,  as  Mendel  showed,  is  the  ratio  found  in  an 
F2  generation  when  the  original  parents  differ  in 
two  pairs  of  alternative  characters,  and  from  the 
proportions  in  which  the  different  forms  of  comb  occur 
we  must  infer  that  the  walnut  contains  both  domi- 
nants, the  rose  and  the  pea  one  dominant  each,  while 
the  single  is  pure  for  both  recessive  characters.  This 
accorded  with  subsequent  breeding  experiments, 
for  the  singles  bred  perfectly  true  as  soon  as  they 
had  once  made  their  appearance." 

The  *'  Mendelians  "  have  devised  an  ingenious 
hypothesis  which  explains  very  plausibly  the  findings 
in  the  above  experiment,  but  to  carry  the  details 
further  would  take  us  too  far  afield.  The  single 
comb,  however,  that  appeared  in  the  experiment 
just  quoted  is  the  sort  of  a  comb  possessed  by  the  wild 
jungle-cock,  which  is  considered  to  be  the  ancestor 
of  all  our  domestic  breeds.  Its  unexpected  ap- 
pearance after  perhaps  thousands  of  generations 
of  fowls  in  which  it  was  absent,  that  is,  replaced 
by  another  variety  of  comb,  is  spoken  of  as  a  re- 
version or  throw-back.  This  puzzling  phenomenon 
is  familiar  to  all  animal  and  plant  breeders.  With 
the  light  which  the  studies  of  Mendelian  inheritance 


VARIATION  AND  HEREDITY  233 

have  thrown  on  the  subject,  it  is  reasonable  to  believe 
that  reversions  are  merely  the  reappearance  of  unit 
characters  which  have  been  latent,  generation  after 
generation,  because  the  proper  combination  of  causal 
factors  or  the  removal  of  an  inhibiting  factor  has 
not  occurred  in  mating.  The  factor  has  been  "  passed 
on,"  however,  from  parent  to  offspring  under  the 
surface,  so  to  speak,  until  the  disturbing  hand  of  the 
experimenter  has  set  new.  combinations  or  removed 
the  inhibitions. 

Sex-limited  Inheritance.  —  Experiments  have 
shown  that,  in  many  cases,  the  correlations  of  somatic 
characters  are  only  explicable  on  the  assumption 
that  the  factors  for  these  characters  are  coupled 
or  linked  together,  and  that  one  is  not  manifest 
without  the  other.  A  striking  example  of  such  a 
correlation  is  found  in  the  fact  that  certain  charac- 
ters are  only  manifest  in  one  sex  although  trans- 
mitted through  both  sexes.  This  involves  the  further 
assumption  that  sex  itself  is  a  Mendelian  allelomorph, 
an  hypothesis  that  had  been  advanced  earlier  on 
cytological  grounds.  Thus,  color-blindness  in  human 
beings  is  a  condition  much  more  prevalent  in  men 
than  in  women,  and  it  has  been  found  that  whereas 
a  woman,  a  daughter  of  a  color-blind  man,  may 
not  reveal  this  character  herself,  she  will  transmit 
it  to  her  sons,  who  will  be  color-blind.  On  the  other 
hand,  her  daughters,  though  capable  of  passing  on  the 
trait,  will,  themselves,  not  be  affected. 

A  more  striking  example  is  found  in  the  little 


234  GENERAL  BIOLOGY 

pomace  fly,  Drosophila,  which  Professor  Morgan 
has  investigated  with  remarkable  results.  This 
little  fly  is  easily  bred  in  captivity  on  fermenting 
bananas,  and  as  its  life  cycle  is  very  short,  it  is 
possible  to  obtain  generation  after  generation  the 
year  round,  and  in  immense  numbers.  A  number 
of  mutations  have  been  obtained  involving  various 
somatic  characters,  such  as  eye-color,  length  of 
wings,  etc.,  which  breed  true.  In  nearly  every 
instance,  however,  these  characters  are  sex-limited ; 
that  is,  although  transmitted  through  either  sex, 
they  become  manifest  in  only  one. 

Economic  Aspects  of  the  Subject.  —  Previous 
to  the  discovery  of  the  laws  of  Mendelian  inheritance, 
animal  and  plant  breeders  were  accustomed  to  choose 
the  types  they  desired  and  "  breed  to "  them ;  in 
other  words,  to  practice  strict  artificial  selection. 
The  fixing  of  the  type  in  this  way  was  a  slow  process 
and  its  stability  always  problematical.  The  appli- 
cation of  the  laws  of  Mendelian  inheritance  has 
changed  the  procedure  of  the  modern  experimental 
breeder  and  has  substituted  certainty  for  previous 
uncertainty  in  the  result,  and  at  the  same  time  greatly 
shortened  the  time  required  to  attain  the  desired 
form.  A  concrete  example  will  make  this  clearer  than 
any  general  discussion.  "  Taken  as  a  whole,  English 
wheats  compare  favorably  with  foreign  ones  in 
respect  of  their  cropping  power.  On  the  other  hand, 
they  have  two  serious  defects.  They  are  liable  to 
suffer  from  the  attacks  of  the  fungus  which  causes 


VARIATION  AND  HEREDITY  235 

rust,  and  they  do  not  bake  into  a  good  loaf.  This 
last  property  depends  upon  the  amount  of  gluten 
present,  and  it  is  the  greater  proportion  of  this  which 
gives  to  the  *  hard  '  foreign  wheat  its  quality  of 
causing  the  loaf  to  rise  well  when  baked.  For  some 
time  it  was  held  that  '  hard  '  wheat  with  a  high 
glutinous  content  could  not  be  grown  in  the  English 
climate,  and  undoubtedly  most  of  the  hard  varieties 
imported  for  trial  deteriorated  greatly  in  a  very  short 
time.  Professor  Biffin  managed  to  obtain  a  hard 
wheat  which  kept  its  qualities  when  grown  in  England. 
But  in  spite  of  the  superior  qualities  of  its  grain  from 
the  baker's  point  of  view,  its  cropping  capacity  was 
too  low  for  it  to  be  grown  profitably  in  competition 
with  English  wheats.  Like  the  latter,  it  was  also 
subject  to  rust.  Among  the  many  varieties  which 
Professor  Biffin  collected  and  grew  for  observation 
he  managed  to  find  one  which  was  completely  im- 
mune to  the  attacks  of  the  rust  fungus,  though  in 
other  respects  it  had  no  desirable  quality  to  recom- 
mend it.  Now  as  the  result  of  an  elaborate  series 
of  investigations  he  was  able  to  show  that  the 
qualities  of  heavy  cropping  capacity,  *  hardness ' 
of  grain,  and  immunity  to  rust  can  all  be  expressed 
in  terms  of  Mendelian  factors.  Having  once  an- 
alyzed his  material,  the  rest  was  comparatively 
simple,  and  in  a  few  years  he  has  been  able  to  build 
up  a  strain  of  wheat  which  combines  the  cropping 
capacity  of  the  best  English  varieties  with  the  hard- 
ness of  the  foreign  kinds,  and  at  the  same  time  is 
completely  immune  to  rust."  ^ 

1  Punnett,  "Mendelism,"  Chapter  XIV. 


236  GENERAL  BIOLOGY 

The  Inheritance  of  Disease.  —  Disease,  to  primi- 
tive man,  must  have  seemed  a  very  mysterious 
thing,  a  fiend  or  evil  god  to  be  placated  or  exorcised. 
Popular  therapeutics  in  some  parts  of  the  world  is 
still  based  upon  such  an  hypothesis.  Even  in  civi- 
lized society,  the  belief  is  still  widely  prevalent  that 
disease  is  a  sort  of  entity,  to  be  gotten  rid  of,  not 
perhaps  by  the  bells  and  the  tom-toms  of  the  medi- 
cine man,  but  by  the  agency  of  drugs.  Our  grand- 
mothers thought  that  boils  were  "  better  out  than 
in,"  and  took  noxious  doses  for  the  purpose  of 
"  cleansing  the  blood  "  of  them.  We  now  know 
that  disease  is  a  process,  not  an  entity.  When  my 
watch  loses  time  because  it  needs  cleaning,  we  might 
say  that  it  runs  "  abnormally."  The  dust  in  the 
works  is  the  cause  of  the  condition,  but  it  itself  is 
not  the  abnormality ;  the  abnormality  is  the  slowing 
down  of  the  movement.  In  the  same  way,  ab- 
normalities in  the  "  running  "  of  the  human  machine 
are  due,  in  the  majority  of  cases,  to  the  presence 
therein  of  foreign  bodies  which  bring  about  the 
altered  conditions.  Usually  these  microbes  are 
bacteria,  though  sometimes  they  are  minute  pro- 
tozoa. They  are  merely  parasites  that,  in  striving 
to  get  a  living  for  themselves  on  the  tissues  of  their 
host,  produce  poisons  as  an  incidental  result  of  their 
activity.  These  poisons  produce  the  alterations 
in  metabolism  we  call  disease.  If  we  can  get  rid 
of  the  active  agents,  the  disease  is  cured,  provided 
the  damage  has  not  gone  too  far.  This  may  be 
done  either  directly  (sterilization)  or  indirectly  by 


VARIATION  AND  HEREDITY  237 

a  reaction  which  we  call  the  development  of  im- 
munity. This  power  has  been  previously  men- 
tioned. There  is  another  class  of  disease-process 
for  which  we  have  not  found  a  causative  agent. 
This  consists  in  the  overemphasis  of  a  metabolic 
process,  itself  normal,  which  only  leads  to  trouble 
when  the  balance  of  the  organism  is  thereby  upset. 
Thus  the  laying  down  of  fat,  which  is  a  normal  and 
necessary  bodily  function,  may  become  overempha- 
sized and  lead  to  a  condition  of  obesity  or  even 
"  fatty  degeneration."  The  local  overdevelopment 
of  connective  and  Epithelial  tissues  produces  tumors, 
etc. 

A  third  class  of  disease  is  concerned  with  the 
nervous  system.  Here  the  difference  between  nor- 
mality and  abnormality  is  very  hard  to  define,  but 
since  the  days  of  witch-burning  and  exorcism  have 
passed,  no  one  believes  that  insanity  is  anything  more 
than  a  condition,  not  a  thing,  whatever  may  be  the 
"  derangements "  of  the  nervous  system  in  which 
it  has  its  origin. 

Owing  perhaps  to  unconscious  suggestion,  due  to 
the  methods  of  insurance  companies,  there  is  a 
rather  strong  popular  belief  in  the  heritability  of 
disease,  a  belief  which,  in  most  cases,  is  unfounded. 
If  we  recall  the  mechanism  of  inheritance  and  the 
fact  that  the  organism  derives  its  individual  heri- 
tage through  a  single  cell,  the  gamete,  and  reflect 
that  disease-conditions  are  practically  always  mani- 
fested in  the  soma,  it  becomes  evident  that  disease 
cannot  be  inherited  unless  there  is  something  passed 


238  GENERAL  BIOLOGY 

on  in  the  germ-cell  itself,  which  is  involved  in 
differentiation  and  later  becomes  a  causative  agent 
for  disease.  Since  we  must  look  upon  disease  as  a 
process,  and  a  process  brought  about,  in  the  majority 
of  cases,  by  the  activity  of  poison-producing,  para- 
sitic bacteria,  it  is  evident  that  there  cannot  be 
anything  in  the  germ-cell,  even  of  a  diseased  person, 
to  produce  the  disease  unless  the  germ-cell  itself 
contains  the  microbe.  With  one  or  two  exceptions 
this  is  not  the  case,  since  the  disease-producing 
bacteria  are  usually  localized  in  certain  regions  of 
the  soma.  Equally  obviously,  disease-processes  that 
are  merely  alterations  of  normal  metabolism  can- 
not, in  themselves,  be  inherited. 

On  the  other  hand,  the  disease-process  is  usually  a 
sort  of  resultant  of  a  parallelogram  of  forces,  of  which 
the  strength  of  the  invading  microbe  is  opposed  to 
the  resistance  of  the  victim.  Now,  not  only  may 
the  resistance,  i.e.  the  power  of  the  body  to  produce 
substances  that  counteract  or  nullify  the  effect  of 
the  toxins,  differ  markedly  in  different  individuals, 
but  also  this  power,  being  a  function  of  the  physical 
organism,  may  be  inherited  in  varying  degrees.  We 
are  therefore  justified  in  speaking  of  the  inheritance 
of  a  tendency  toward  certain  diseases,  although  what 
we  really  mean  is  the  inheritance  of  a  low  resistance 
on  the  part  of  the  body  to  the  disease,  and  the 
consequent  ease  with  which  the  disease  is  contracted. 
For  in  every  case  there  must  be  a  new  infection  in 
each  generation.  It  is  believed  that  even  leprosy, 
one  of  the  most  dreaded  of  human  afflictions,  the 


VARIATION  AND  HEREDITY  239 

"  taint  "  of  which,  in  popular  thinking,  is  indissolubly 
linked  with  the  idea  of  its  transmission,  is  not  really 
inherited,^  but  is  in  each  case  a  new  infection. 

The  variation  in  the  degree  of  specific  immunity 
or  resistance  to  certain  diseases  may  be  racial  as 
well  as  individual.  Thus  it  is  frequently  stated 
that  various  children's  diseases,  such  as  mumps, 
measles,  etc.,  which  with  us  are  annoying,  but  by 
no  means  dangerous,  affections,  when  introduced 
among  isolated  populations,  such  as  those  of  the 
South  Sea  Islands,  develop  a  virulence  unknown 
among  us,  and  may  carry  off  whole  communities. 
The  "  resistance  "  of  strains  or  varieties  of  plants 
to  disease  is  quite  comparable  to  that  of  animals. 
The  inheritance  of  resistance  to  the  fungus-disease, 
"  rust,"  to  which  wheat  is  subject,  has  just  been 
mentioned.  Without  doubt  a  similar  immunity- 
factor  for  various  human  diseases  will  be  discovered 
in  the  near  future. 

Inheritance  of  Defects.  —  Nevertheless,  observa- 
tion teaches  us  that  certain  human  conditions  usually 
classed  as  diseases  are  indeed  directly  inherited. 
Deafness  and  color-blindness  are  found  to  be  trans- 
mitted in  the  same  way  that  hair-color  or  eye-color  is. 
Haemophilia,  the  name  given  to  the  condition  in 
which  the  blood  does  not  clot  and  on  account  of 
which  the  victim  may  bleed  to  death  from  a  scratch, 

i"It  is  quite  certain  that  the  children  of  lepers  born  out  of  leper 
districts^in  England  or  the  United  States,  for  example,  never  inherit 
it"  —  Quoted  by  Thompson  from  Hutchinson  in  AUbutt's  "  System  of 
Medicine,"  I,  1896. 


240  GENERAL  BIOLOGY 

is  another  example.  So  is  hyperdactylism,  in  which 
there  is  an  abnormal  number  of  toes  or  fingers. 
But  these  are  physiological  and  structural  characters, 
and  not  disease-processes  in  the  true  sense.  They 
are  merely  characters  unsuitable  for  preservation. 
We  would  not  discover  their  hereditary  character 
except  that  they  are  preserved  through  the  altruistic 
endeavor  of  man,  just  as  De  Vries  in  his  garden 
preserved  the  more  delicate  mutants  of  his  evening 
primroses  which  otherwise  would  have  been  crowded 
out  of  existence  in  competition  with  the  sturdier 
races.  Such  atypical  racial  characters  are  probably 
much  more  numerous  than  we  are  accustomed  to 
think.  They  include  not  only  structural  and  physio- 
logical items,  but  psychical  and  moral  ones  as  well. 
While  insanity,  of  itself,  cannot  be  inherited,  yet  the 
structural  basis  for  insanity,  that  is,  an  abnormal 
nervous  system,  is  inherited  just  the  same  as  any 
other  structural  character.  Thus  "  innate  deprav- 
ity "  is  by  no  means  a  figure  of  speech,  and  "  feeble- 
mindedness "  is  very  persistently  inherited  with  all 
its  accompaniments  of  mental  and  moral  obliquity. 

Eugenics.  —  Man,  in  contrast  to  the  rest  of  organ- 
ized nature,  largely  controls  his  environment  instead 
of  being  controlled  by  it.  Nature's  eliminations  are 
frequently  nullified  by  his  altruism.  In  preserving 
his  "  unfit,"  however,  he  is  imposing  a  very  heavy 
burden  of  support  upon  the  fit  and  normal  members 
of  the  race.  It  has  been  found  that  these  so-called 
unfit  members  of  society  are  fully  as  productive  in 


VARIATION  AND  HEREDITY  241 

rearing  offspring  as  other  classes.  More  than  this, 
statistics  show  that  the  children  of  the  most  superior 
classes  of  humanity,  both  morally  and  intellectually, 
are  very  largely  outnumbered  by  the  mediocre  or 
inferior.  In  other  words,  the  superior  classes,  the 
cream  of  the  race,  are  not  continuing  their  heritage, 
and  were  it  not  for  constant  reenforcement  from  the 
"  lower  "  grades  of  society,  the  so-called  intellectual 
element  would  soon  be  self -exterminated.  Pro- 
fessor Pearson  estimates  that  twenty-five  per  cent  of 
the  mothers  in  Great  Britain  produce  fifty  per  cent  of 
the  next  generation.  It  is  a  matter  of  much  moment 
to  civilized  man  from  what  classes  the  coming  gener- 
ations are  to  be  born.  So  far  as  statistics  may  be 
depended  upon,  it  would  seem  that  the  proportion  of 
defectives,  comprising  all  sorts  of  persons  who,  on 
account  of  physical,  moral,  or  mental  abnormalities, 
are  a  burden  to  society,  is  steadily  and  rapidly 
increasing. 

Much  attention  is  being  given  nowadays  to  this 
problem  and  its  solution.  Francis  Galton,  who  first 
called  attention  to  the  subject,  invented  the  word 
"Eugenics,"  which  he  defined  as  "  the  science  of  being 
well  born."  If  man  proves  himself  able  to  cope  with 
the  problem,  it  will  be  because  he  has  analyzed  and 
tested  the  facts  of  heredity,  the  knowledge  of  which, 
alone  will  enable  him  to  improve  the  human  race, 
or  prevent  it  from  slipping  back  from  the  present 
standards  of  civilization. 


CHAPTER  VIII 
ORGANIC  RESPONSE 

Environment.  —  Except  in  abstract  thought,  a 
living  organism  cannot  be  dissociated  from  the  rest 
of  the  universe  of  which  it  forms  a  part.  This 
"  rest  "  we  call  the  environment.  It  includes  all 
external  matter,  the  presenqe  or  absence  of  which, 
or  the  alteration  of  which,  produces  any  sort  of  a 
change  in  the  organism  itself.  The  sj^atial  limits 
included  under  the  term  environment  are  wholly 
relative.  Thus,  the  soil-environment  of  a  tree  is 
very  limited  in  extent,  whereas  the  sun,  in  spite  of  its 
very  great  distance  from  the  earth,  forms  a  very 
essential  part  of  its  environment  as  well  as  that  of  all 
living  forms.  The  community  in  which  a  man  lives 
forms  a  significant  part  of  his  organic  environment, 
since  the  presence  or  absence  of  other  people  affects 
and  conditions  his  own  actions.  A  few  centuries 
ago  such  a  social  environment  would  have  been  very 
limited  in  extent  on  account  of  the  isolation  of  com- 
munities. Nowadays,  thanks  to  telegraph  and 
newspaper,  human  activities  in  any  part  of  the  world 
may  alter  or  affect  the  actions  of  any  one ;  hence  this 
sort  of  environment  has  greatly  extended. 

In  a  more  limited  sense,  however,  we  usually 
apply  the  term  environment  to  an  organism's  immedi- 

242 


ORGANIC  RESPONSE  243 

ate  surroundings,  —  the  water,  for  instance,  in  which  a 
fish  swims,  including  in  it  such  factors  as  temperature, 
light,  chemical  substances,  pressure,  etc.  We  have 
seen  that  the  most  impressive  characteristic  of  living 
matter  is  its  ceaseless  flux  and  flow.  This,  however, 
is  not  only  true  of  such  unstable  things  as  living 
organisms,  but  is  also  true,  according  to  the  story  of 
Geology,  of  mountains  and  continents.  The  only 
permanent  thing  in  the  Universe,  organic  or  inor- 
ganic, is  its  eternal  changefulness.  We  have  seen, 
too,  that  the  existence  of  an  organism  or  the  existence 
of  an  aggregate  of  organisms  is  dependent  upon  a 
most  delicate  balance  of  innumerable  "  forces." 
But  these  forces  nray  act  external  to  the  organism  as 
well  as  internal ;  that  is,  they  may  be  of  the  environ- 
ment. Any  change  in  the  environment  may  thus 
produce  a  corresponding  change  in  the  balance  of  the 
organism.  Such  an  environmental  change  may  be 
called  a  stimulus.  The  readjustment  in  the  organ- 
ism due  to  such  environmental  change  may  be  of 
two  kinds.  It  may  be  a  simple  alteration  of  relations 
comparable  to  the  crystallization  of  water  with  the 
lowering  of  the  temperature  to  the  freezing  point, 
in  which  case  we  speak  of  it  as  a  physical  response. 
On  the  other  hand,  it  may  involve  the  release  of 
energy  stored  up  in  the  protoplasm,  which  manifests 
itself  in  ways  peculiar  to  and  dependent  upon  the 
organization  of  the  latter.  This  second  type  of 
response  is  called  the  organic  response  or  reaction. 
The  stimulus  in  such  a  case  may  be  compared  with 
the    impact    of    the    hammer    which    explodes    the 


244  GENERAL  BIOLOGY 

cartridge.  The  most  usual  environmental  changes 
producing  organic  responses  are  those  of  tempera- 
ture, chemical  conditions,  light,  electricity,  and 
mechanical  contact.  This  fundamental  ability  of 
the  living  matter  to  respond  to  stimuli  is  known  as 
Irritability. 

The  Usual  Conditions  of  Environment 

Temperature.  —  For  all  living  things  a  certain 
degree  of  warmth  is  requisite,  but  organisms  vary 
greatly  in  this  regard.  For  every  organism  there  is 
an  optimum  temperature  at  which  it  grows  and 
thrives  best,  and  this  is  apt  to  be  the  usual  tempera- 
ture in  which  the  organism  naturally  occurs.  Plants 
and  animals  of  the  tropics  require  a  higher  degree  of 
heat  than  do  those  of  temperate  zones,  and  when 
we  transplant  them  they  need  "  hothouses  "  in 
order  that  they  may  thrive.  There  is  also  for  each 
sort  of  organism  a  minimum  and  a  maximum  tem- 
perature. Since  protoplasm  is  so  largely  made  up 
of  water,  its  temperature  cannot  fall  below  32° 
Fahrenheit,  if  life  is  to  be  maintained.^  As  we  shall 
see  further  on,  the  protoplasm  of  some  kinds  of 
animals  and  plants  may  be  protected  against  such 
conditions  and  hence  may  be  unaffected  by  freezing 
temperature.  It  is  equally  obvious  that  the  tem- 
perature of  boiling  water  will  destroy  living  matter  by 

^  The  death  of  the  living  matter  is  due,  in  all  probability,  not  so 
much  to  the  alteration  of  the  temperature  per  se  as  to  the  fact  that  the 
freezing  of  the  water  into  needles  of  ice  rends  and  tears  the  fundamental 
structures  of  the  cell  beyond  repair. 


ORGANIC  RESPONSE  245 

coagulating  its  proteids,  although  authentic  instances 
are  known  of  lower  plants  and  animals  living  in  hot 
springs  at  a  very  little  below  boiling  temperature. 

Light.  —  As  we  have  seen,  sunlight  is  an  absolutely 
essential  condition  of  life  for  all  green  plants,  and 
hence  secondarily  for  all  organisms,  and  the  effect 
on  plants  of  the  alteration  or  absence  of  light  is  very 
marked.  But  sunlight  is  a  destructive  agent  for 
many  groups  of  molds  and  bacteria,  particularly  of 
many  pathogenic  forms,  and  in  consequence  man 
and  the  higher  animals  are  very  dependent  on  its 
cleansing  and  purifying  influence.  Sunlight  destroys 
the  "  germs  "  that  otherwise  would  threaten  them 
with  extermination  through  disease. 

Chemical  Environment.  —  Since  the  nature  of  the 
soil  varies  with  its  chemical  composition,  the  organ- 
isms living  in  the  soil  are  affected  so  far  as  these 
chemical  substances  are  in  solution.  As  combina- 
tions and  recombinations  of  chemical  substances  are 
easily  brought  about  and  constantly  occurring  in  the 
soil,  the  organism  in  general  is  very  delicately  ad- 
justed to  its  chemical  environment  and  influenced 
by  it. 

In  the  air  we  have  both  chemical  and  physical 
agencies  to  take  into  consideration.  Not  only  its 
pressure  (some  15  lbs.  to  the  square  inch),  but  more 
particularly  its  movements,  have  an  important 
general  influence  on  the  organism,  both  plant  and 
animal.  In  addition,  the  composition  of  the  atmos- 
phere has  an  important  and  more  significant  bearing 


246  GENERAL  BIOLOGY 

on  the  conditions  of  life  for  all  organisms.  The 
oxygen,  which  constitutes  20  per  cent  of  ordinary  air,  is 
absolutely  essentially  for  practically  all  living  things.^ 
The  nitrogen,  as  we  have  seen,  is  the  raw  material 
which  the  soil-bacteria  transform  into  salts  available 
for  plants.  The  CO2  is  the  source  of  the  carbohy- 
drates. Almost  equally  important  in  its  effect  on 
organisms,  particularly  plants,  is  the  amount  of 
moisture  in  the  atmosphere.  The  difference  in 
appearance  between  the  vegetation  of  a  dry  arid 
region  and  one  supplied  with  abundant  moisture  is 
familiar  to  every  one.  Of  course,  there  is  a  very 
intimate  relation  between  the  moisture  of  the  soil  and 
that  of  the  adjacent  air,  but  the  general  appearance 
of  a  region  is  largely  determined  by  the  moisture 
brought  to  it  by  the  wind  in  the  form  of  rain-clouds. 

The  Nature  of  Organic  Response.  —  From  one 
point  of  view  an  organism  may  be  considered  to  be 
always  in  a  condition  of  stimulation,  which  we  call 
tone.  There  must  always  be  a  certain  amount  of 
heat,  for  example,  to  make  life-conditions  possible. 
The  increase  or  decrease  of  the  external  temperature, 
however,  causes  a  readjustment  on  the  part  of  the 
organism,  which  is  the  evident  or  perceptible  reac- 
tion. This  response  may  be  of  two  sorts.  On  the 
one  hand,  the  vital  phenomena  may  be  changed  in 
character,  or  qualitatively,  especially  if  the  reaction 
is  a  very  violent  one,  but  more  often  they  are  merely 

^  With  the  exception  of  the  anaerobic  bacteria,  which  get  their  oxygen 
in  other  ways. 


ORGANIC   RESPONSE  247 

changed  quantitatively,  i.e.  increased  or  decreased, 
whence  the  reactions  are  sometimes  termed  excita- 
tions and  depressions.  The  chemical  stimulus  of 
substances  known  as  narcotics  produces  a  depression 
of  protoplasm.  Increase  of  heat  produces  excita- 
tion, decrease  (cold),  depression.  For  each  kind  of 
protoplasm  there  are  minimal  and  maximal  limits  of 
stimulation  within  which  it  displays  its  characteris- 
tic vital  phenomena,  but  the  nature  of  the  reaction 
to  any  stimulus  whatever  is  of  course  dependent  upon 
the  peculiar  characteristics  of  protoplasm  itself;  in 
other  words,  upon  its  organic  make-up.  In  propor- 
tion as  protoplasm  becomes  specialized  the  nature 
of  its  response  becomes  more  and  more  restricted. 
Any  stimulus  applied  to  a  muscle,  e.g.,  causes  the 
same  sort  of  a  response  (contraction)  whether  the 
excitant  be  electrical,  thermal,  mechanical,  or  chemi- 
cal. This  specificity  of  response  is  sometimes  known 
as  the  law  oj  specific  energy. 

Electric  Response.  —  In  the  majority  of  cases  we 
recognize  the  existence  of  a  stimulus  and  a  response 
by  a  change  of  form,  but  excitation  and  response 
may  be  present  without  being  evident  to  our  senses. 
A  nerve  when  stimulated  shows  no  change  in  itself 
even  though  it  transfer  its  stimulation  to  the  muscle 
with  which  it  is  connected.  Nevertheless  if  we  lay 
across  a  nerve  two  electrodes,  connected  through  a 
galvanometer,  and  stimulate  the  nerve  by  pinching 
it,  we  will  see  by  the  deflection  of  the  needle  of  the 
galvanometer  that  an  electrical  change  has  taken 


248  GENERAL  BIOLOGY 

place  in  the  protoplasm  of  the  nerve. ^  Similar 
electrical  changes  may  be  observed  in  other  tissues 
as  well,  but  only  when  alive;  a  dead  nerve  or  a 
narcotized  nerve  gives  no  such  reaction.  This 
electric  response,  which  may  be  demonstrated  also  in 
plants,  has  been  referred  to  as  the  critical  sign  of  life. 
But  Professor  Bose  of  Calcutta  has  shown  that  metals 
give  a  quite  similar  response,  provided  all  strains  have 
been  removed  by  annealing.  A  tin  wire  thus  treated, 
and  "  stinnilated  "  by  mechanical,  thermal,  or 
chemical  means,  will  show  the  same  phenomena  of 
response  that  a  nerve  or  a  plant  does.  Bose's  con- 
clusions, which  are  very  far-reaching,  are  that  there 
is  no  essential  difference  between  organic  and  inor- 
ganic matter,  with  respect  to  response.  The  differ- 
ence between  living  and  dead  protoplasm,  he  thinks, 
is  probably  the  condition  of  the  absence  or  presence 
of  permanent  molecular  strains,  which  is  another 
way  of  saying  that  dead  protoplasm  is  relatively 
stable,  whereas  the  primary  characteristic  of  living 
matter  is  its  lability.  The  primary  distinction 
between  the  response  of  living  and  of  non-living 
matter,  as  already  pointed  out,  lies  in  the  fact  that, 
in  the  former,  the  energy  released  by  the  response  may 
be  many  times  greater  than  that  of  the  stimulus. 

Individual  Response  to  XJnsymmetrical  Stimuli.  — 
Heretofore  we  have  considered  environmental  stim- 
uli in  a  general  way,  as  affecting  all  parts  of  the 

1  It  has  also  recently  been  demonstrated  that  a  stimulated  nerve  gives 
off  CO2,  indicating  the  nature  of  the  chemical  reactions  taking  place 
within  its  substance. 


ORGANIC  RESPONSE 


249 


Fig.  85.  —  Orientation  of  Nasturtiums  toward  the  source  of  light : 
/,  two  plants  growing  in  diffuse  daylight;  //,  the  same  plants  after  a  few 
hours'  exposure  to  light  from  the  window  (at  the  left). 


250  GENERAL   BIOLOGY 

organism  at  once.  It  is  evident,  however,  that 
many  classes  of  stimuli  —  for  example,  rays  of  light, 
and  currents  of  electricity  —  may  affect  only  one  part 
of  the  organism  at  a  time,  producing  necessarily 
an  unsymmetrical  stimulation.  If  the  more  strongly 
stimulated  side  were  concerned  in  movement,  a  change 
in  direction  would  result.  If,  for  instance,  the 
stimulation  should  produce  an  increase  of  muscular 
or  protoplasmic  contraction  on  that  side,  the  effect 
would  be  to  turn  the  organism  around  until  it  should 
reach  a  position  in  which  both  sides  were  stimulated 
equally ;  in  other  words,  to  orient  the  organism.  If, 
now,  the  progressive  movements  still  kept  up,  the 
direction  of  the  progression  of  the  organism  would 
be  toward  the  source  of  stimulation.  If  the  stimulus 
were  a  depressant  instead  of  an  excitant,  the  orienta- 
tion would  be  reversed ;  that  is,  the  reaction  would  be 
negative. 

Recent  experimental  work  has  revealed  an  ex- 
traordinary range  of  responses  of  this  sort  among  all 
kinds  of  animals  and  plants.  The  most  familiar 
perhaps  is  the  tendency  of  green  plants  to  face  the 
source  of  light.  In  some  cases,  as  in  the  sunflower, 
the  plant-head  follows  the  sun  as  a  compass-needle 
the  magnet.  The  behavior  of  the  unicellular 
"  swarm  spores  "  or  of  the  motile  gametes  of  the 
lower  algae  is  quite  similar.  They  gather  in  a  mass  at 
the  side  of  the  dish  exposed  to  the  window.  In  the 
insect  world  the  fascination  of  the  moth  for  the  flame 
has  become  proverbial.  At  first  glance  one  might 
think  that  a  wide  gap  exists  between  the  mechan- 


ORGANIC  RESPONSE 


251 


ical  bending  of  a  plant  and  the  movement  of 
flying  organism,  but  careful  experiment  has 
that  it  is  not  curiosity  that 
leads  the  moth  to  the  candle, 
nor  the  exercise  of  a  choice  of 
any  sort,  but  rather  a  mechani- 
cal orientation  which  the  insect 
is  powerless  to  control.  Again, 
if  microorganisms,  such  as  Para- 
mecia,  be  placed  in  a  narrow 
trough  of  water  through  which 
a  weak  current  of  electricity  is 
passed,  they  will  orient  them- 
selves in  the  direction  of  the 
current  and  swim  to  one  or  the 
other  of  the  two  poles  (usually 
the  negative).  This  action  is 
equally  as  unvolitional  as  the 
response  to  the  source  of  light 
and  may  be  reversed  as  often 
as  the  direction  of  the  current 
is  reversed. 

We  know  very  little  of  the 
nature  of  the  mechanism  under- 
lying this  phenomenon.      Nor- 


a  free- 
shown 


252  GENERAL  BIOLOGY 

mally  the  cilia  of  Paramecium  beat  in  a  continuous 
and  specific  manner,  and  the  course  of  the  organism 
through  the  water  is  determined  by  the  shape  of  the 
cell-body  itself.  It  is  as  incapable  of  modifying  this 
movement  as  a  man  in  a  rowboat  would  be  of  alter- 
ing that  of  the  boat's  movement.  In  the  latter  case 
the  oarsman  can  only  propel  his  craft  forward, 
backward,  or  in  a  curve,  depending  upon  his  pulling 
the  oars,  pushing  them  forward,  pulling  on  one  oar 
more  than  on  the  other,  or  finally  pulling  on  one  oar 
and  pushing  on  the  other.  Indeed  these  movements 
reduce  to  two,  pushing  and  pulling.  Now  if  we 
postulate  that  areas  of  the  ciliated  surface  of 
Paramecium  are  capable  of  local  and  independent 
stimulation,  then  the  stronger  or  weaker  beat  of  the 
cilia  due  to  such  stimulation  in  certain  regions  would 
orient  the  organism,  its  own  movements  determining 
the  direction  of  its  progression. 

The  mechanical  nature  of  such  tropic  responses  is 
evident  in  cases  in  which  the  "  tropism  "  may  be 
arbitrarily  reversed  by  the  experimenter.  Thus 
many  small  animals,  like  the  swarm-spores  men- 
tioned, gather  on  the  side  of  the  dish  toward  the  light. 
That  this  is  not  the  expression  of  a  choice  or  prefer- 
ence of  the  organism  for  one  sort  of  illumination  in 
contrast  to  another  is  demonstrated  by  the  fact  that 
changing  the  temperature,  increasing  the  salinity 
(of  sea- water),  or  even  agitating  the  water,  may  bring 
about  a  reversal  of  the  response.  In  such  a  way  we 
alter  the  physiological  state  of  the  organism  and  cause 
it  to  react  in  a  different  manner. 


ORGANIC  RESPONSE  253 

Some  biologists  believe  that  the  majority  of  reac- 
tions to  stimuli,  such  as  light,  heat,  diffusion  of 
chemicals,  electricity,  etc.,  are  based  upon  some  such 
automatic  mechanism.  We  should  be  very  cautious, 
in  any  event,  in  interpreting  such  movements  in 
terms  of  human  experience,  and  ascribing  choice 
and  volition  to  organisms  that  exhibit  such  a  response. 
On  the  other  hand,  the  careful  study  of  the  behavior 
of  a  number  of  microorganisms  has  shown  that  the 
same  individual  will  not  react  at  all  times  in  the 
same  way  to  the  same  stimulus,  which  it  would  be 
compelled  to  do  if  the  response  were  absolutely 
mechanical.  The  nature  of  the  response  is  depend- 
ent upon  the  "  physiological  state  "  of  the  organism 
at  the  time  of  stimulation.  But  this  does  not  mean 
that  such  responses  are  purposive,  even  if  they  are 
to  the  advantage  of  the  organism  possessing  them. 

Adaptive  Response.  —  We  are  familiar  with  many 
ways  in  which  our  own  bodies  accommodate  them- 
selves automatically  to  environmental  change.  The 
greater  demand  we  make  upon  a  muscle,  the  more  the 
muscle  increases  and  grows  to  meet  that  demand. 
Those  parts  of  the  skin  which  are  exposed  to  friction 
develop  horny,  protecting  calluses.  If,  through 
disease,  one  kidney  becomes  ineffective  or  function- 
less,  the  remaining  one  grows  larger  (hypertrophy)  in 
order  the  better  to  carry  out  the  double  burden  laid 
upon  it.  These  examples  indicate  how  very  plastic 
even  'such  an  organism  as  highly  specialized  man 
may  be.     The  changes  mentioned,  and  many  other 


254 


GENERAL  BIOLOGY 


similar  ones,  are  responses  to  external  stimuli  of 
various  sorts,  although  we  have  no  knowledge  of  the 
means  by  which  the  stimulus  brings  about  its  result- 
ant reaction.  Such  reactions,  however,  differ  in  an 
important  way  from  those  mentioned  in  the  previous 
section  in  that  they  are  advantageous  to  the  organism 

exhibiting  them,  often  even  to 
the  extent  of  determining  its 
preservation  from  destruction. 
A  striking  example  of  adaptive 
response  to  environmental 
change  is  found  in  the  African 
chameleon  and  in  its  Ameri- 
can representative,  the  lizard 
Anolis,  of  Florida  and  the 
Southern  States.  This  crea- 
ture displays  a  wonderful 
capacity  for  color  change. 
Normally  bronze,  it  runs 
through  olive  to  pale  green  or 
turquoise  blue.  This  change 
is  produced  by  the  migration, 
up  and  down  through  the 
dermis,  of  black  pigment  cells, 
are  factors  that  bring  about 
Just  to  what  extent  the  color 
changes  of  Anolis  are  advantageous  to  the  lizards 
in  causing  them  to  resemble  their  backgrounds  is 
a  little  difficult  to  estimate.  There  is  not  much 
question,  however,  that  in  some  other  forms,  in 
which  the  changes  are  accomplished   more   slowly, 


Fig.  87.  —  Anolis,  the 
American  chameleon. — (Cole- 
man.) 

Both    heat  and  light 
these  color  changes. 


ORGANIC  RESPONSE  ^55 

such  as  the  shore  crabs,  the  tree-frogs,  and  the 
flat-fishes,  the  color  change  is  protective  and 
brought  about  under  the  direct  influence  of  the 
environment.  Another  striking  effect  of  tempera- 
ture is  that  produced  by  cold  upon  hairy  animals. 
The  small  shaggy  ponies  of  Shetland  and  Iceland, 
while  in  their  native  land,  are  provided  not  only 
with  the  ordinary  hair  characteristic  of  horses  every- 
where, but  also  with  a  dense  woolly  fur  underneath, 
which  serves  to  keep  them  warm  in  very  cold  weather. 
When  brought  into  a  warmer  climate,  however,  one 
year  suffices  to  shed  this  dense  coat,  and  thereafter 
the  hair  is  no  different  from  that  of  other  horses. 

Immunity.  —  One  of  the  most  striking  examples  of 
individual  adaptation  is  found  in  the  manner  in  which 
the  higher  organisms  (or  at  least  the  warm-blooded 
ones)  react  against  infectious  diseases.  An  organism 
that  is  non-susceptible  to  a  specific  microbic  disease 
is  said  to  be  immune  to  that  disease.  This  immunity 
may  be  "  natural  "  or  it  may  be  "  acquired."  It  is 
probable  that  all  natural  immunity  has  been  acquired 
during  the  lifetime  of  the  race,  through  the  auto- 
matic elimination  of  the  non-immune  individuals. 
Artificial  immunity,  on  the  other  hand,  is  the  individ- 
ual affair  of  the  organism.  In  pathology,  two  kinds 
are  recognized :  active  immunity  in  which  the  tissues 
of  the  body  react  directly  against  the  toxins  of  the 
invading  bacteria,  and  passive  immunity,  which  is 
confCTred  upon  the  individual  by  the  injections  of 
blood-serum  from  another  actively  immune  animal. 


^56  GENERAL  BIOLOGY 

The  latter  is  the  basis  of  the  "  anti-toxins  "  that  are 
now  so  extensively  used  in  combating  infectious 
diseases.  It  is  only  the  active  immunity  that  con- 
cerns us  here. 

When  disease-producing  bacteria  gain  entrance  to 
the  body  and  begin  to  multiply,  the  products  of  their 
metabolism  produce  a  poisoning  or  "  toxic  "  effect, 
either  upon  the  whole  organism  or  upon  certain 
organs,  such  as  the  nervous  system.  The  body 
doubtless  reacts  in  several  ways.  Thus  it  has  been 
shown  that  the  leucocytes  devour  the  invading 
microbes  and  thus  defend  the  organism  from  attack. 
But  the  most  striking  phenomenon  is  this :  that  the 
presence  of  the  toxins  stimulates  the  body  to  produce 
substances  called  "  anti-bodies,"  which  are  carried 
by  the  blood  and  appear  to  combine  with  the  toxins 
and  nullify  their  poisonous  effect.  Once  having  been 
stimulated,  the  body  continues  to  produce  the  anti- 
bodies after  all  disease-symptoms  have  disappeared. 
In  this  way,  it  is  immune  against  a  second  attack, 
whereas  it  was  susceptible  before.^  Moreover, 
the  blood-serum  carrying  these  anti-bodies  may  be 
drawn  from  one  animal  and  injected  into  another, 
thereby  conferring  the  passive  immunity  mentioned 
above, 

Morphogenetic  Response.  —  In  the  previous  sec- 
tion, with  a  few  exceptions,  we  have  considered  the 
reactions  of  the  individual  to  stimuli  that  are,  as  a 
rule,  of  short  duration,  or  at  any  rate  produce  no 

^  In  the  human  species  it  will  be  recalled  that  there  are  some  diseases 
of  which  this  is  not  true. 


ORGANIC  RESPONSE  257 

permanent  alteration  of  the  structure  or  form  of  the 
organism  after  their  cessation.  There  is  another 
type  of  response,  however,  which  results  in  a  perma- 
nent structural  change  and  may  therefore  be  called 
morphogenetic. 

Non-adaptive  Morphogenetic  Response. — A  great 
many  experiments  have  been  tried  by  different 
observers  to  test  the  effect  of  various  sudden  environ- 
mental changes  on  developing  organisms.  Abnor- 
mal conditions  of  heat  and  cold,  humidity  and 
dryness,  food,  etc.,  have  been  found  to  produce  re- 
markable alterations  in  the  color  patterns  of  va- 
rious butterflies  and  beetles.  Standfuss,  Fischer,  and 
others  have  found  that  if  chrysalids  of  moths  or 
butterflies  are  kept  in  an  ice-box  prior  to  emergence, 
the  perfect  insects  develop  niany  "  aberrations," 
i.e.  the  color  markings  of  the  butterflies  are  different 
from  the  usual  type,  often  to  a  striking  degree.  It 
is  very  interesting  to  discover  that  these  artificially 
produced  aberrations  are  very  similar  to  such  as 
occur  in  nature  and  have  received  names  in  collec- 
tions. Particularly  is  this  true  of  such  species  as 
have  "  winter  "  and  "  summer  "  forms.  In  most 
cases,  doubtless,  the  effect  of  the  change  in  tempera- 
ture is  to  directly  alter  the  chemical  processes  taking 
place  in  the  color-producing  cells  of  the  integument. 
As  a  general  rule  the  effect  of  a  slightly  increased 
temperature  is  to  hasten  the  activity  of  the  color- 
producing  enzymes  and  hence  to  increase  the  inten- 
sity of  the  color.     With  slightly  lowered  temperature 


258  GENERAL  BIOLOGY 

the  reverse  is  the  case.  It  has  been  amply  demon- 
strated that  there  is  nothing  specific  in  the  effect 
of  these  variations  in  the  environmental  complex. 
Extreme  heat  and  extreme  cold  will  most  often  pro- 
duce identical  effects.  Tower,  in  the  course  of  long 
experimentation  with  the  potato  beetle,  has  shown 
that  the  exposure  of  the  developing  insect  to  moist 
conditions  produces  a  dark  (melanic)  beetle,  whereas 
the  relative  absence  of  moisture  produces  a  cor- 
responding lack  of  pigment  (albinism) .  It  is  inter- 
esting to  discover  that  the  beetles  found  in  nature 
in  dry  countries  such  as  the  southwestern  United 
States  are  albinic,  whereas  those  found  in  clayey  soils 
in  northwestern  United  States  are  melanic.  The 
same  conditions  evidently  have  produced  the  same 
result,  both  in  the  laboratory  and  in  a  state  of  nature. 

Influence  of  Food.  —  It  is  well  known  that  many 
flowers  may  be  artificially  colored  by  allowing  them 
to  soak  up  various  dyes.  The  presence  or  absence  of 
numerous  other  chemical  substances  in  the  soil  also 
produces  a  difference  in  habit  of  growth,  size,  color, 
etc.  A  considerable  amount  of  lime,  e.g.,  will  induce 
a  hairiness  on  leaves  and  stems  together  with  a  more 
abundant  foliage.  It  has  been  observed,  too,  that 
caterpillars  fed  on  a  different  food  plant  from  that 
to  which  they  are  accustomed  sometimes  develop 
into  moths  and  butterflies  with  quite  different  color 
markings  from  the  normal,  although  there  is  no 
relation  between  the  color  markings  and  the  specific 
food  plant.     The  color  of  the  caterpillar  in  many 


ORGANIC  RESPONSE  259 

species,  however,  is  due  to  the  color  of  the  food  plant. 
The  egg  of  the  honey  bee  develops  either  into  a  queen 
or  into  a  worker  according  to  the  nature  and  quantity 
of  the  food  that  is  supplied  the  larva  by  the  attending 
bees. 

General  Adaptation.  —  To  the  man  who  stops  to 
look  below  the  surface  of  things  one  of  the  most 
wonderful  aspects  of  nature  is  the  apparently  perfect 
way  in  which  all  living  organisms  are  suited  to  the 
particular  sort  of  environment  in  which  they  are 
found.  As  soon  as  one  realizes  that  the  environment 
is  not  a  permanent,  changeless  sort  of  a  thing,  but  is 
as  plastic  and  impermanent  as  the  organisms  them- 
selves, the  marvel  grows  that  so  many  forms  of  life 
should  be  or  should  have  become  adapted  to  a 
particular  sort  of  environment  at  a  given  time. 
And  the  more  thought  that  is  given  the  matter,  the 
more  it  is  realized  that  this  adjustment  is  one  of  the 
fundamental  problems  of  biology.  We  have  seen 
that  the  individual,  as  a  rule,  responds  very  quickly 
to  any  alteration  of  environmental  conditions,  but 
experiment  also  shows  conclusively  that  the  response 
is  only  individual  and  that  the  abstraction  we  call 
race,  or  species,  is  unaffected  thereby.  How,  then, 
does  it  come  about  that  the  species  is  so  admirably 
adapted  .f^  Two  answers  have  been  offered  to  this 
problem,  the  Darwinian  and  the  Lamarckian,  and 
this  theoretical  aspect  of  the  phenomena  we  shall 
consider  in  the  next  chapter,  contenting  ourselves 
for  the  present  with  reviewing  some  of  the  more 


260 


GENERAL  BIOLOGY 


striking  examples  of  adaptation.  It  must  not  be 
forgotten,  however,  that  all  species,  except,  perhaps, 
such  as  are  in  the  process  of  extinction,  are  very 
adequately  adapted  to  the  particular  environment 
in  which  they  are  found,  else  they  would  not  be 
existent  at  all. 

Some  Types  of  Adaptation 
Aquatic  Organisms. — All  the  animals  that  live  in 
the  water  show  a  certain  general  form  and  structure 


Fig.  88.  —  Types  of  aquatic  adaptation,  showing  convergence  toward 
a  boat-like  structure  in  diverse  and  unrelated  groups  of  animals.  The 
outline  opposite  each  figure  is  a  diagrammatic  cross-section  :  A,  diving 
beetle ;  B,  herring ;  C,  mud  turtle ;  D,  killer  whale  (a  warm-blooded 
animal  most  nearly  akin,  perhaps,  to  the  cud-chewing  mammals  or 
perhaps  to  the  ant-eaters). 

that  enables  them  to  go  the  more  easily  through  the 
resistant  medium  in  which  they  live.  If  we  examine 
a  diving  beetle  (fig.  88,  a),  a  fish  (fig.  88,  b),  a  turtle 


Fig.  89.  —  Convergence  toward  a  globular  form  in  unrelated  pelagic 
organisms :  A,  Pelagonemertes,  a  Nemertean  worm  which  floats  on  the 
open  sea;  its  congeners  are  very  elongate  and  "worm-like"  and  live 
near  the  shore ;  p,  proboscis ;  g,  proboscis  sheath ;  c,  cerebral  ganglia ; 

B,  Trochosphcera,  a  Rotifer,  allied  to  the  worms,  which  swims  by  means 
of  a  circlet  of  cilia,  c^,  c^ ;  C,  a  free-swimming  medusa  (Bougainvillea) 
typical  of  the  phylum  Ccelenterata;  D,  a  Ctenophor  (Beroe),  a  member 
of  a  group  that  is  but  distantly  related  to  the  Ccelenterata,  with 
which,-however,  it  is  usually  included  in  textbooks ;  ot,  balancing  organ ; 
tr,  funnel  of  the  digestive  canal.  —  (A  and  D,  from  Sedgwick;  B  and 

C,  from  Parker  and  Haswell.) 


262  GENERAL  BIOLOGY 

(fig.  88,  c),  or  a  whale  (fig.  88,  d),  we  see  that  in  each 
there  is  a  swelling,  curving  contour  like  that  of  a 
boat,  which  offers  the  least  resistance  to  movement 
through  the  water.  In  addition,  a  sharp  cutting 
edge  or  keel  is  sometimes  developed.  Although  the 
forms  mentioned  have  very  little  else  in  common,  and 
although  their  "  relatives  "  differ  much  from  one  an- 
other, yet  in  accommodation  to  a  free-swimming 
habit  they  have  "  adopted  "  each  a  general  type 
of  structure.  This  sort  of  a  resemblance  between 
organisms  otherwise  unrelated,  in  adaptation  to  a 
common  environment,  is  called  Convergence.  In 
those  marine  forms  which  float  semipassively  in  the 
open  sea  (pelagic  organisms)  convergence  may  be 
carried  to  a  greater  extreme.  (Compare  fig.  89.) 
Here  two  factors  have  been  of  significance  in  each 
type :  (a)  the  necessity  for  decrease  in  specific 
gravity  and  (6)  a  minimum  increase  of  surface  for 
the  maximum  of  bulk.  A  sphere  is  the  shape  that 
ideally  fits  the  latter  condition,  and  we  find  that 
pelagic  organisms  in  general  tend  toward  a  spherical 
form.  The  former  condition  has  brought  about 
an  elimination  of  a  great  part  of  the  solids  in  the 
organism,  and  the  result  is  a  form  which  contains  a 
very  large  per  cent  of  water  and  is  usually  semi- 
transparent. 

Aerial  Adaptations.  —  The  development  of  wing- 
like structures  has  enabled  representatives  of  various 
diverse  groups  of  organisms  to  maintain  themselves 
in  the  air.     The  whole  group  of  insects  is  preemi- 


ORGANIC  RESPONSE  263 

nently  of  this  class,  and  of  the  vertebrates,  the  birds, 
although  the  bats,  among  the  mammals,  share  with 
the  birds  the  conquest  of  the  air.  But  in  some  other 
groups  as  well,  special  types  of  aerial  or  semi- 
aerial  forms  are  thus  endowed.  In  the  flying-fishes 
the  pectoral  fins  are  long  and  expanded,  so  that  the 


■f 


Fig.  90.  —  Two  kinds  of  Flying  Fishes.  These  fishes  escape  from 
their  enemies  by  leaping  into  the  air  and  sailing  long  distances.  —  (From 
Jordan  and  Kellogg.) 

fish,  after  attaining  headway  in  the  water,  can  shoot 
above  the  surface  and,  spreading  the  fins,  soar  like  an 
aeroplane  for  long  distances. 

Subterranean  Adaptations.  —  In  the  keen  com- 
petition that  exists  among  the  various  types  of 
animals  for  a  foothold  and  a  chance  to  propagate 
their  kind,  not  only  the  surface  of  the  earth,  the  air, 
and  the  water  have  become  filled  with  life,  but,  in  the 


264  GENERAL  BIOLOGY 

course  of  evolution,  various  different  and  wholly 
unrelated  forms  have  become  adapted  to  live  under 
the  earth  itself.  Omitting  the  Protozoa  in  the  soil, 
of  which,  as  yet,  we  know  but  little,  and  the  animals 
that  merely  burrow  in  the  ground  for  protection,  we 
find  that  these  various  and  diverse  forms  have  all 
become  modified  in  much  the  same  way.  In  the 
water  of  subterranean  caverns  we  usually  find 
various    Crustacea     (crayfish    and    their    relatives). 


Fig.  91.  —  A  deep-sea  fish  {Stomias  boa)  with  luminous  organs  (photo- 
phores)  along  the  sides.  —  (From  Hickson,  after  Filhol.) 


salamanders,  and  fishes.  In  every  case  these  are 
blind,  the  same  conditions  apparently  having  pro- 
duced the  same  consequence  in  all.  In  the  absence 
of  light  eyes  are  useless,  and  Nature  brings  about 
their  atrophy  (just  how,  is  a  matter  of  speculation 
upon  which  biologists  are  not  agreed).  On  the 
other  hand,  in  the  abysses  of  the  sea,  the  weak 
light  that  filters  down  from  the  surface  is  appar- 
ently sufficient  to  make  vision  possible,  and  we  do 


ORGANIC  RESPONSE  265 

not  find  blind  fishes  in  such  a  situation.  In  many 
of  these  forms  "  phosphorescent  "  organs  or  photo- 
phores  are  developed  which  function  either  as  lures 
or  as  recognition  lights  or  for  the  purpose  of  actually 
furnishing  an  artificial  light  for  the  creature's  needs. 
The  ants  are  relatives  of  the  wasps  and  bees,  al- 
though their  lives  are  mostly  spent  underground  in 
the  complicated  galleries  which  they  excavate. 
Their  well-known  structure  is  evidently  well  adapted 


Fig.  92.  —  Ant-guests  :  Flies  that  live  in  ant-nests  and  are  adapted 
for  life  underground ;  the  one  at  the  right  is  a  male,  the  other  two 
females.  —  (From  Kellogg,  after  Wheeler.) 


to  the  peculiar  life  of  the  ant-nest.  It  is  of  great 
interest  to  discover  that  representatives  of  other 
groups  of  insects  are  found  in  ant-nests,  and  spend 
their  lives  underground.  These  include  beetles, 
flies,  crickets,  roaches,  and  other  types,  but  all  of 
them,  in  accommodating  themselves  to  the  conditions 
of  ant-life,  have  become  altered  (racially,  of  course, 
not  individually)  to  such  an  extent  that  they  have 
come  to  resemble  ants  and  have  lost  their  resemblance 
to  related  species  of  their  own  kind. 


266  GENERAL  BIOLOGY 

Protective  Adaptation.  —  Some  one  has  classed 
free-living  animals  in  two  groups,  the  preyers  and  the 
preyed-upon.  Of  course,  many  types  would  be- 
long to  both  classes,  but  some  find  it  easier  to  run 
away  than  to  fight,  and  their  chief  characteristics 
are  those  which  serve  for  protection  rather  than  for 
aggression.  Some,  like  the  turtles  or  the  majority 
of  the  mollusks,  possess  an  armor  within  which  they 


Fig.  93.  —  Hermit-crab  (Pagurus)  in  a  snail  shell,  with  a  sea-anemone 
attached  to  the  shell.  — (From  Jordan  and  Kellogg,  after  Hertwig.) 

can  withdraw,  or  like  the  porcupine  or  the  spiny 
puffer-fish,  an  armor  that  wards  off  the  aggressor. 
Others,  like  the  hermit  crab,  which  utilizes  the  pro- 
tection of  empty  snail-shells  and  is  modified  in 
accordance  with  that  peculiar  mode  of  life,  get  their 
armor  second-hand.  An  interesting  adaptation  for 
protection  is  that  of  the  cuttle-fish  and  "  devil-fish  " 
(Octopus),    which    secretes    quantities    of    an    inky 


ORGANIC  RESPONSE  267 

fluid.  When  one  of  these  mollusks  is  attacked,  it 
beclouds  the  water  by  spurting  out  this  fluid,  and, 
under  cover  of  this  protection,  makes  its  escape. 

Protective  Coloration.  -7  Most  striking  of  all  adap- 
tations whereby  animals  escape  their  enemies  is 
that  of  protective  coloration.  A  bird  darts  into  a 
thicket  and,  strain  our  eyes  as  we  may,  we  cannot 
see  where  it  has  gone.  A  grasshopper  is  started  up 
and  whirs  away,  conspicuously  visible  on  account 
of  the  brilliant  coloration  of  its  wings,  only  to  dis- 
appear utterly  from  sight  as  it  drops  back  again 
into  the  grass.  The  bright  colors  are  covered  up,  and 
the  coloration  of  the  outer  surface  of  the  body  merges 
into  that  of  the  creature's  surroundings.  Not  the 
least  effective  element  of  its  disappearance  is  the 
sudden  and  bewildering  contrast  between  the  con- 
spicuousness  of  the  insect's  appearance  at  one 
moment  and  its  inconspicuousness  at  another. 
This  condition  extends  to  many  unrelated  types  of 
animals.  Along  the  white  sandy  stretches  of  the 
sea-shore,  or  the  bare  rocks  and  sands  of  the  desert, 
practically  all  animal  life  partakes  of  the  same 
whitish,  inconspicuous  ground-color.  In  the  leafy 
depths  of  the  forest  the  inhabitants  are  likely  to  be 
green  as  well.  Familiar  examples  of  forms  that  show 
such  protective  coloration  are  the  tree-frog  and  the 
katydid,  or  the  fishes  that  live  among  the  sea-weeds 
in  a  tide-pool.  Often  an  animal  that  appears  to  be 
most  conspicuously  marked  when  isolated  on  a 
museum  shelf  merges  perfectly  into  its  environment 


268  GENERAL  BIOLOGY 

when  it  occupies  its  natural  surroundings.  Exam- 
ples of  this  are  grouse  and  woodcock  and  even 
domestic  poultry. 

Specific  Resemblance.  —  The  sort  of  protective 
resemblance  just  described  is  of  a  general  nature; 
that  is,  the  animal  merges  into  the  general  back- 
ground of  its  surroundings  without  resembling  in 
particular  any  one  element  of  its  environment. 
Sometimes,    however,    the    protective    resemblance 


Fig.  94.  —  The  Katydid  (Microcentrum) .  The  wings  are  colored  and 
veined  like  the  leaves  of  the  vegetation  in  the  midst  of  which  the  insect 
hides.  —  (After  Riley,  from  Kellogg.) 

takes  a  more  specific  form,  and  an  animal  is  found 
to  resemble  some  specific  object  in  its  environment 
rather  than  the  general  background.  Thus,  the 
katydid's  wings  are  veined  in  such  a  way  as  to  resem- 
ble very  closely  a  green  leaf,  and  a  common  moth- 
larva  not  only  mimics  a  dry  twig  in  color  and 
shape,  but  has  the  habit  of  extending  its  body 
out  into  the  air  from  a  branch  so  as  to  make  the 
imitation  almost  perfect.  Such  examples  may 
be  found  almost  anywhere  in  the  woods  and  fields, 
but  they  appear  to  be  especially  plentiful  in  the 
tropics. 


ORGANIC  RESPONSE  269 

Aggressive  Coloration.  —  While  many  animals  seem 
to  escape  detection  by  making  themselves  as  incon- 
spicuous as  possible,  others  would  appear  to  court 
observation.  Wasps  and  bumble  bees  fly  about  un- 
molested, and  many  brilliantly  striped  and  spotted 
bugs  take  no  pains  to  conceal  their  conspicuous 
presence.  This  habit  is  likely  due  to  the  fact 
that  they  are  not  unprotected.  The  experience  of 
stings  in  the  one  case  or  of  a  bad  taste  and  odor 
in  the  other  may  make  such  an  impression  on  an 
insect-eating  animal  that  similar  insects  would 
thereafter  be  avoided.  The  bright  and  conspicuous 
colors  would  thus  be  a  sort  of  advertisement  or 
danger  signal.  The  more  conspicuous  the  color 
pattern,  the  less  likely  would  the  insect  be  eaten 
by  mistake  and  the  more  valuable  its  livery.  This 
would  of  course  not  help  the  insect  so  unfortunate 
as  to  be  attacked,  but  the  others  of  his  kind  would 
profit  by  the  mistake.  The  victim,  so  to  speak, 
would  be  sacrificed  for  the  sake  of  educating  its 
enemies  to  leave  its  relatives  alone. 

Mimicry.  —  The  advantage  enjoyed  by  such  an 
advertisement  is  in  some  cases  shared  by  other 
species  that  have  no  natural  means  of  defense. 
Thus  many  bees  and  wasps  are  "  imitated "  so 
closely  by  certain  flies  as  to  make  it  almost  impossible 
to  tell  one  from  another  when  on  the  wing.  The 
same  kind  of  natural  fraud  is  found  among  butter- 
flies. A  familiar  example  of  widespread  occurrence 
in  America  is  the  mimicry  of  the  Monarch  or  Milk. 


Fig.  95.  —  The  mimicking  of  the  inedible  Monarch  butterfly  bj^  the  edible  Viceroy. 
The  upper  figure  is  the  Monarch  (Anosia  plexippus) ;  the  middle  figure,  the  Viceroy 
(Ldminitis  archipptis)  ;  the  lower  figure  is  another  species  of  the  genus  Liminitis  (L.  i 
arthemis)  which  is  supposed  to  be  the  type  from  which  archippus  has  been  derived.— 
(From  Jordan  and  Kellogg.)  I 


ORGANIC  RESPONSE  271 

weed  butterfly,  Anosia,  by  the  viceroy  butterfly, 
Liminitis  (see  plate).  The  larva  of  the  former 
feeds  upon  the  milkweeds,  and  it  is  supposed  that 
its  body-fluids  partake  of  the  disagreeable  taste  of 
the  food-plant,  and  that  on  this  account,  being 
conspicuously  colored,  it  is  avoided  by  butterfly- 
hunting  birds.  Another  group  of  butterflies  not  at 
all  closely  related  to  Anosia  is  the  genus  Liminitis, 
of  which  some  seven  or  eight  species  are  found  in 
North  America.  All  of  these  but  two  are  marked 
with  a  white  stripe  across  each  wing  and  are  very 
different  in  appearance  from  Anosia.  The  impor- 
tant exception  is  Liminitis  archippus,  which  resembles 
Anosia  so  closely  in  general  appearance  that  unless 
one  were  an  entomologist  he  would  hardly  think 
of  discriminating  the  two  when  on  the  wing.  The 
advantage  to  Liminitis  archippus  of  its  masquerade 
may  be  inferred  from  the  fact  that  whereas  Anosia 
ranges  over  nearly  the  whole  of  North  America  and 
the  viceroy  also,  the  other  species  of  Liminitis  have 
very  restricted  and  comparatively  narrow  ranges. 

For  such  mimicry  to  be  successful  it  is  necessary 
that  the  mimic  should  vary  widely  from  the  type 
of  its  congeners  and  that  it  should  be  much  less 
abundant  than  the  "  model."  Natural  Selection  ^ 
has  usually  been  called  upon  to  explain  such  a  phe- 
nomenon, but  the  theory  offers  many  difficulties  in 
such  a  case,  and  scientists  are  far  from  agreed  upon 
an  explanation.  The  fact  of  mimicry  is,  however, 
indisputable. 

^  See  next  chapter. 


272 


GENERAL  BIOLOGY 


The  Care  of  the  Young.  —  Another  type  of  adapta- 
tion of  very  great  value  to  the  species  in  which  it  is 
developed  is  that  involved  in  parental  solicitude  and 
care  for  progeny.  Among  the  lower  forms,  par- 
ticularly the  invertebrates,  the  rule  is  for  the  mother 
animal  to  lay  an  enormous  number  of  eggs  and 
^  ^  trust  to  luck,  so  to  speak,  that 

/     J.      \  enough  survive  to  maintain  the 

j\KSS^^  position  of  the  species.    In  many 

g^j^^mKyX         ^^  t^^  insects,  however,  this  is 
^  jMSS^^  ^^    ^^^  ^t  ^^^  ^^^  case.     The  workers 
^  N^Ma^SB^^    in  a  beehive  or  an  ant  hill  attend 
and  feed  the  helpless  grubs  until 
they  are  ready  to  shift  for  them- 
selves.     Among   certain  of  the 
wasps,  the  mother,  although  de- 
.r.  ^]^\  96  —Water  Bug  pending  for  her  own  food  upon 

(Zaitha).     Male    carrying  •  i    p  n 

eggs  on  its  back.  —  (From  ucctar  Sipped  irom  flowcrs,  yet 
Jordan  and  Kellogg.)  catches  flics  for  her  camivorous 

young.  Finally  we  have  a  situation  such  as  is  to  be 
observed  in  Zaitha,  one  of  the  water  bugs,  in  which 
the  female  seizes  the  weaker  male,  and  in  spite  of 
his  struggles,  glues  her  eggs  all  over  his  back,  trans- 
forming him,  for  the  time  being,  into  a  nurse. 

A  somewhat  similar  example  is  to  be  found  in  the 
Surinam  toad,  Pipa.  In  this  species,  the  female 
places  the  eggs  on  her  own  back,  where  they  sink  in, 
forming  little  pits  that  close  over.  When  ready  to 
hatch,  the  covers  break  off  and  the  baby  toads 
wriggle  down  to  water  to  complete  their  metamor- 
phosis.    In  another  kind  of  toad,  Alytes,  the  male 


^W' 


ORGANIC  RESPONSE  273 

wraps  the  strings  of  eggs  around  his  legs  and  carries 
them  until  they  hatch.  One  of  the  most  remarkable 
of  these  habits  is  that  of  certain  fishes  which  carry 
their  eggs  in  the  mouth  until  they  hatch.  The  large 
**  Gaff  Topsail  "  catfish  common  along  the  Atlantic 
Coast,    and   other   sea   catfishes,    have   this   habit, 


Fig.  97.  —  The  Surinam  Toad  (Pipa) .  — (From  "  A  Textbook  in  Gen- 
eral Zoology,"  copyright  1907,  by  Glenn  W.  Herrick.  Permission  of  the 
American  Book  Co.,  Publishers.) 

the  eggs  being  carried  from  the  time  they  are  laid 
until  the  young  fish  is  able  to  swim  about  by  itself, 
at  least  three  weeks,  and  in  some  cases  probably 
much  longer.  As  in  the  other  cases,  it  is  the  male 
that,  does  this,  and  during  this  period,  of  course, 
he  takes  no  food. 

The  most  familiar  example  of  parental  care  for 


274  GENERAL  BIOLOGY 

the  young  is  found  in  the  birds,  nearly  all  of  which 
display  this  characteristic  to  a  greater  or  less  degree. 
Some  sea  birds  lay  their  eggs  on  the  bare  rocks  and 
pay  no  more  attention  to  them  thereafter.  The 
majority  of  birds,  however,  build  some  sort  of  a 
nest,  and  in  some  cases  this  is  of  elaborate  design. 
In  many  cases  bot^i  male  and  female  share  the  labor 
of  brooding  the  eggs  and  bringing  food  to  the  fledg- 
lings. 

Environmental  Adaptations  of  Plants.  —  Since 
plants  have  not  the  advantage  animals  enjoy  of 
moving  from  place  to  place,  they  are  profoundly 
modified  by  the  nature  of  the  soil  in  which  they 
grow.  If  this  is  rich  and  fertile,  their  growth  is 
luxuriant;  if  dry  and  poor,  they  are  stunted,  and  the 
aspect  of  two  regions  may  differ  very  greatly  on 
this  account.  Temperature  also  plays  an  important 
part,  the  vegetation  of  the  arctics  and  of  alpine 
regions  being  also  stunted  in  comparison  with  the 
more  luxuriant  growth  of  warmer  regions.  But 
the  abundance  or  scarcity  of  water  probably  plays 
the  most  significant  role  in  determining  the  character 
of  the  vegetation  in  any  region. 

As  previously  noted,  the  plant  world  probably 
took  its  origin  in  the  water  and  secondarily  migrated 
to  the  land.  Large  groups,  however,  are  still 
confined  to  the  former  medium.  These  constitute 
the  familiar  type  of  the  water  plants  or  hydrophytes. 
They  are  characterized  by  soft  and  succulent  tis- 
sues  with  scant  supporting  tissue,   since  they  are 


ORGANIC  RESPONSE  275 

supported  by  the  medium.  Some  float  freely  on  the 
surface,  such  as  the  familiar  "  duck-weed  "  or  Lemna. 
Others  are  entirely  submerged,  though  often  rooted 
to  the  bottom.  Such  plants  have  a  great  develop- 
ment of  leaflike  or  chlorophyll-bearing  tissue, 
which  makes  up  for  the  deficient  supply  of  sunlight 
beneath   the   surface   of   the   water.     Some   of   the 


Fig.  98.  —  Hydrophytic  vegetation. 

marine  algae  of  this  class  grow  to  enormous  dimen- 
sions and  develop  floats  which  buoy  them  up. 
At  the  other  extreme  from  the  hydrophytes  we  have 
the  xerophytes,  comprising  vegetation  characteristic 
of  deserts,  where  water  is  extremely  scanty.  Only 
those  forms  that  are  able  to  avail  themselves  of  the 
meager  supply  of  water  in  such  a  situation,  have  been 


276 


GENERAL  BIOLOGY 


ORGANIC   RESPONSE  277 

able  to  exist  and  maintain  a  foothold.  Such  plants 
usually  absorb  very  rapidly  such  rain  as  falls,  or 
else  have  very  long  tap-roots  that  penetrate  deeply 
into  the  soil  and  take  up  the  maximum  amount 
of  water.  Thus  one  of  the  morning  glories  {Con- 
volvulus), which  grows  on  the  dry  western  plains, 
instead  of  developing  into  a  delicate,  weak-rooted, 
climbing  vine,  as  do  most  of  the  Convolvuli,  grows  as 
a  sort  of  bush,  a  foot  or  so  high,  and  sends  a  huge 
root  down  twenty  or  thirty  feet  into  the  soil. 

Another  advantageous  structure  found  in  plants 
of  such  regions  is  one  which  brings  about  the  storing 
up  of  water.  Succulents  or  fleshy  plants  have  a 
large  amount  of  parenchyma  tissue  which  holds 
water  as  a  sponge.  This  is  true  of  many  species  of 
cactus.  In  addition,  such  plants  usually  have  the 
exposed  surface  reduced  to  a  minimum.  This 
retards  and  lessens  the  loss  of  water  through  evapora- 
tion. In  Cacti  of  various  sorts  the  stem  is  succulent, 
and  the  leaves,  as  such,  are  absent.  Xerophytes 
are  also  adapted  to  economize  the  water  they  have 
in  store  and  undergo  long  periods  of  drought.  An 
extreme  example  is  the  well-known  "  Resurrection 
plant  "  {Selaginella),  which  grows  on  the  sides  of 
rocky  cliffs  in  Mexico  and  may  be  pulled  up  and 
dried  for  years,  only  to  uncurl  and  freshen  up  again 
in  a  few  hours  when  placed  in  a  saucer  of  water. 

Midway  between  the  two  extremes  just  described, 
the  hydrophytes  and  the  xerophytes,  are  the  plants 
we  are  most  familiar  with,  the  Mesophytes.  These 
are  adapted  to  various  degrees  of  temperature  or 


278  GENERAL  BIOLOGY 

of  changes  of  moisture,  growing  most  luxuriantly 
in  the  greatest  degree  of  warmth  and  moisture  as 
in  the  humid  climate  of  the  tropics.  In  the  abun- 
dant vegetation  of  these  regions  there  has  arisen 
a  sharp  "  struggle  for  light  "  which  has  brought  into 
being  a  great  variety  of  climbing-plants  and  vines, 
and  of  aerial  plants  that  depend  for  water  upon 
what  their  hair-covered  aerial  rootlets  draw  from  the 
moist  atmosphere. 

Adaptations  for  Seed  Dispersal. — Unable  to  move 
from  the  place  in  which  they  are  rooted,  plants  are 
nevertheless  able  to  spread  their  various  species 
over  available  territory  with  great  rapidity,  chiefly 
on  account  of  special  adaptations  in  connection  with 
the  dispersal  of  their  seeds. 

Many  seeds  are  formed  to  float  in  the  wind  and 
are  transported  long  distances  in  this  way.  A 
familiar  example  is  "  thistle  down  "  and  the  cottony 
seed  of  the  milkweed.  Often  the  seed  is  winged, 
as  in  the  case  of  the  maple  or  the  catalpa.  In  some 
cases,  the  whole  plant  is  transported  by  the  winds. 
On  the  western  plains,  the  Russian  thistle  or  "  tum- 
ble-weed "  (there  are  several  sorts)  grows  into  a 
large  globular  bush  which  breaks  off  close  to  the 
ground  when  ripe  and  goes  rolling  across  the  prairie, 
scattering  its  seeds  as  it  goes,  until  brought  to  a 
standstill  by  a  fence. 

Other  seeds  are  wonderfully  adapted  to  stick  to 
anything  that  touches  them,  particularly  the  furry 
coat  of  animals.     The  "  beggar's  lice  "  (Desmodium) 


ORGANIC   RESPONSE  279 

has  a  flat  seed,  covered  with  stiff  hooks  which  cling 
tenaciously.  Another  familiar  example  is  the  sand- 
bur  (Cenchrus),  with  its  pod  covered  with  spines. 
The  usefulness  of  bright-colored  and  succulent 
fruits  to  the  plant  which  produces  them  is  supposed 
to  consist  in  the  fact  that  birds  which  eat  them 
transport  the  hard  seeds  within  the  fruit  to  a  distance 
from  the  parent  tree. 

Associations  of  Animals.  —  Among  the  most  prim- 
itive animals  the  individual  lives  its  own  life  without 
reference  to  any  other  individual,  although  favorable 
conditions  or  abundance  of  food  often  brings  numbers 
of  the  same  species  together  in  a  given  locality.  In 
many  species,  however,  we  frequently  find  animals 
^sseciating  themselves  together  in  groups,  apparently 
from  mere  love  of  company  or  gregariousness.  But 
the  mutual  advantages  of  self-protection  afforded 
by  such  a  relation  are  not  inconsiderable.  Many 
fishes  swim  in  "  schools  "  of  thousands  of  individuals. 
Deer  and  antelope  feed  in  herds,  and  jiiany  kinds 
of  birds  in  flocks.  Under  such  circumstances  it  is 
very  difficult  for  an  enemy  to  approach,  without 
some  member  of  the  company  giving  the  alarm. 
The  same  sort  of  association  also  serves  an  aggres- 
sive purpose.  Wolves  and  dogs  hunt  in  packs, 
and  it  is  related  that  the  pelicans  of  the  Gulf  of 
Mexico  form  in  a  circle,  narrowing  toward  the 
shore,  and  by  splashing  the  water,  drive  in  the  fish 
to  shallow  water,  where  they  are  easily  captured. 
The  social  instincts  of  such  insects  as  the  ants  and 


280  GENERAL  BIOLOGY 

bees  are  deservedly  famous,  the  division  of  labor 
involved  in  the  getting  and  storing  of  food  and  the 
rearing  of  young  being  carried  to  a  point  that  equals 
the  standards  of  human  savages.  Instances  of  sim- 
pler associations  of  individuals  of  the  same  species 
might  be  multiplied  indefinitely. 

Commensalism.  —  The  relations  that  exist  be- 
tween different  species  in  the  same  environment 
are  usually  of  a  quite  different  character.  Animals 
are,  as  a  rule,  either  indifferent  to  or  hostile  to 
individuals  of  other  species.  There  are  many 
exceptions,  however,  to  this  statement.  Species 
of  widely  separated  groups  have  in  many  cases 
entered  into  partnerships  more  or  less  intimate. 
In  the  simplest  form  of  this  sort  of  association,  one 
species  attaches  itself  in  some  way  to  a  larger  or 
stronger  one  to  profit  by  the  protection  afforded 
or  the  food  supplied.  An  example  is  foimd  in  the 
suckfish  (Remora),  which  has  the  dorsal  fin  modified 
into  a  sucking  disk,  by  which  it  attaches  itself  to 
sharks  or  other  voracious  fishes^  living  on  the  frag- 
ments that  escape  its  companion's  mouth.  In 
another  case,  a  delicate  transparent  fish  called 
Fierasfer  is  found  in  the  interior  of  Holothurians 
(sea  cucumbers)  or  between  the  gills  of  mussels,  where 
it  gets  the  darkness  and  protection  that  is  thereby 
afforded.  A  more  familiar  example  is  the  relation 
that  exists  between  a  crustacean,  Caprella,  and  va- 
rious hydroids,  particularly  Pennaria.  Here  pro- 
tective resemblance  comes  into  play  to  such  an  extent 


ORGANIC  RESPONSE 


281 


that  it  is  very  difficult  to  discern  the  Caprella  on  the 
hydroid  colony  even  when  one  is  looking  for  them. 
To  the  safety  afforded  by  this  protective  resemblance 
may  be  added  the  protection  of  the  stinging  nettle 
cells  of  the  hydroid.  Such  a  relation,  which  profits 
but  one  party 
to  the  associa- 
tion and  on  the 
other  hand  does 
not  injure  the 
other,  is  termed 
mutualism  or 
commensalism. 
In  other,  cases 
both  sides  profit 
by  the  connec- 
tion. A  very 
familiar  exam- 
ple is  the  rela- 
tion that  exists 
between  the 
aphids  and 
ants.  The 
former  secrete 
a  jelly-like  sub- 
stance called 
"  honey-dew,"  which  the  ants  are  very  fond  of,  and 
in  order  to  maintain  a  supply,  the  ants  keep  the 
aphids  in  flocks,  as  men  do  cattle,  tending  them, 
carrying  them  back  if  they  stray,  and  "  milking  " 
them  by  stroking  them  with  their  antennae,  thereby 


Fig.  100.  —  Rose  Aphids  visited  by  ants,  natural 
size  from  life.  —  (From  Kellogg.) 


282  GENERAL  BIOLOGY 

hastening  the  secretion  of  the  desired  substance. 
The  ants  will  usually  fiercely  attack  trespassers,  and 
the  aphids  are  thus  assured  of  a  protection  which 
nature  has  denied  them,  in  return  for  which  they 
supply  their  keepers  with  food.  An  instance  of  the 
persistence  with  which  the  ants  look  after  their 
"  cattle,"  is  afforded  by  a  discovery  of  Professor 
Forbes.  In  the  Mississippi  Valley,  a  member  of 
the  aphid  iamiiy  called  the  "  corn-root  louse " 
infests  the  roots  of  maize.  Its  eggs  are  laid  in  the 
ground  in  autumn  and  hatch  out  the  following 
spring.  Rotation  of  crops  and  other  methods  of 
combating  the  pest  were  unsuccessful,  until  it  was 
discovered  that  the  little  brown  ant  (Lasius  hriin- 
neus)  takes  the  aphids  when  they  hatch  and 
before  there  are  corn-roots  to  feed  on,  and  "  with 
great  solicitude  carefully  places  them  on  the  roots 
of  certain  kinds  of  knot-weed  (Setaria  and  Poly- 
gonum) which  grow  in  the  field,  and  there  protects 
them  until  the  corn  germinates." 

In  other  cases  the  association  is  not  so  advantage- 
ous. Certain  species  profit  by  the  industry  of  others 
without  giving  an  adequate  return.  A  classic 
example  is  the  cuckoo,  which  lays  its  eggs  in  other 
birds'  nests,  to  be  brooded  and  reared  vicariously. 
The  bee-moth,  if  it  can  escape  the  sentry  at  the  door, 
enters  the  hive  and  lays  its  eggs,  appropriating  room 
and  food  that  the  bees  need  for  themselves.  In 
such  a  case  the  "  host  "  is  injured,  but  only  indi- 
rectly. It  is  only  a  step,  however,  to  a  condition 
in  which  the  host  is  directly  injured  by  the  activity 


ORGANIC  RESPONSE  283 

of  the  species  which  attaches  itself  to  it.  Such  a 
condition  is  called  parasitism,  and  it  is  very  wide- 
spread throughout  the  animal  kingdom.  Rather 
arbitrarily,  zoologists  discriminate  between  parasites 
that  attach  themselves  to  the  outside  of  their  host 
(ectoparasites),  and  those  that  dwell  within  the  body 
of  the  host  (endoparasites) .  ^ 

Parasitism  in  Protozoa.  —  A  large  and  important 
group  of  the  Protozoa  (the  Sporozoa)  has  adopted 
an  exclusively  parasitic  mode  of  existence.  The 
most  familiar  as  well  as  the  most  important  example 
is  the  Plasmodium  malarice,  which  is  parasitic  in 
the  blood  of  man  and  also  in  a  certain  species  of 
mosquito  {Anopheles  maculipennis) .  This  intro- 
duces us  at  once  to  a  remarkable  feature  of  para- 
sitism which  we  find  in  nearly  all  the  groups,  the 
alternation  of  two  very  different  hosts  in  which  por- 
tions of  the  life-cycle  of  the  parasite  are  spent. 

The  malaria-producing  parasite  lives  in  the  red 
blood-cells  of  man  (an  allied  form  is  found  also  in 
birds).  It  reproduces  rapidly  by  multiple  fission 
(sporulation) ,  each  new  individual  attacking  a 
corpuscle,  until,  in  extreme  cases,  nearly  every 
blood -cell  in  the  victim  is  parasitized.  The  charac- 
teristic fever  of  malaria  is  apparently  due  to  the 
liberation  of  some  toxin  produced  by  the  parasites 
simultaneously  with  their  sporulation.  In  addition 
to  the  asexual  reproduction  just  described  there  are 
also  produced  gametes,  of  two  sizes,  analogous  to 
sperm   and  ova.     The  vegetative  phase  has   some- 


Fig.  101.  —  The  Life  Cycle  of  the  Malaria-parasite.  1,  the  organism  (sporozoite)  in- 
troduced by  the  mosquito  bite  into  the  human  blood ;  2,  the  same  free  in  the  blood ; 
3-5,  changes  which  it  undergoes  within  the  red  blood-cells;  6,  spores  (merozoites) 
formed  by  division,  with  the  attendant  destruction  of  the  blood-cell ;  this  cycle  2-6  may 
be  repeated  many  times,  but  eventually  spores  develop  in  the  blood-cells  into  sexual 
forms ;  7,  8,  9,  development  of  a  macrogamete  or  female  element ;  la,  8a,  9o,  96,  devel- 
opment of  microgamete  or  male  element ;  10,  conjugation  of  the  two  sexual  cells ;  11,  the 
resultant  zygote ;  12,  zygote  that  has  entered  the  body  of  the  mosquito  and  has  worked  its 
way  to  the  wall  of  the  latter's  stomach,  —  it  lies  at  the  base  of  two  cells;  13-16,  trans- 
formation of  zygote  into  an  oocyst  filled  with  spores;  17,  adult  female  malarial  mos- 
quito (Anopheles),  head  of  the  male  below;  19,  larva;  20,  pupa;  21,  stomach  of 
mosquito,  showing  cysts  produced  by  the  parasites ;  22,  cross-section  of  salivarj'  gland 
of  mosquito,  showing  the  cells  full  of  spores  which  have  wandered  in  from  the  cysts  qf 
21  and  are  now  ready  to  pass  into  the  blood  with  the  bite  of  the  mosquito. 


ORGANIC  RESPONSE  285 

what  the  appearance  of  a  tiny  amoeba,  but  in  the 
sexual  phase  the  parasite  withdraws  to  one  side 
of  the  blood  corpuscle  in  a  characteristic  crescent- 
like shape.  It  is  now  called  the  gametocyte.  If 
now  an  Anopheles  mosquito  bites  a  man  suffering 
from  malaria,  the  parasitized  blood  is  drawn  up 
into  the  stomach  of  the  mosquito,  where  the  gametes 
free  themselves  and  conjugate.  The  active  zygotes 
then  penetrate  the  stomach  wall  and  become  en- 
cysted, giving  rise  to  an  enormous  number  of  motile 
spores.  These  find  their  way  to  the  salivary  gland 
of  the  mosquito,  and  thence  into  the  blood  of  the 
man  whom  the  mosquito  bites.  Once  introduced, 
they  attack  the  red  blood-cells,  and  the  cycle  begins 
again. 

Parasitism  in  Worms.  —  The  heterogeneous  group 
of  "  worms  "  includes  several  grand  divisions  that 
are  almost  wholly  parasitic  in  their  mode  of  life 
and  have  become  correspondingly  modified  in  struc- 
ture. An  American  species  that  has  recently  be- 
come notorious  is  the  "hookworm"  {Necator 
americanus)  that  is  widely  distributed  over  the 
Southern  states.  The  larvae  of  this  microscopic 
worm  lives  in  the  ground  and  finds  entrance  through 
the  skin  of  its  human  victim.  It  then  makes  its 
way  through  heart  and  lungs  to  the  alimentary 
canal,  where  it  attaches  itself  to  the  wall,  feeds 
on  the  blood  of  its  host,  and  reproduces  prolifically. 
As  a  result,  the  host  soon  begins  to  show  the  effects 
of  malnutrition  and  anaemia.     This  takes  the  form 


286 


GENERAL  BIOLOGY 


of  a  loss  of  energy  and  vigor  and  a  "  shiftless- 
ness  "  for  which  the  "  poor  white  "  of  the  South  is 
famous.  The  group  to  which  this  worm  belongs  is 
known  as  the  Nematoda.  They  are  all  round, 
often  thread-like,  with  a  thick  cuticle,  and  they  in- 
fest many  kinds  of  animals  besides  man.  Another 
intestinal  parasite  of  a  different  sort  is  the  tapeworm, 
a  representative  of  the   "  flat  worms  "  or  Platodes. 


Fig. 


102.  —  Parasitic  worm  in  the  body-cavity  of  a  stickleback ;  B,  the 
worm  extended,  enlarged  1|  times.  —  (Gamble.) 


This  is  a  ribbon-like  worm  of  which  the  body  is 
broken  up  into  a  great  many  segments,  all  of  them, 
except  the  head  segment,  practically  similar.  This 
worm,  like  the  hookworm,  attaches  itself  to  the  lining 
of  the  alimentary  canal  of  its  (vertebrate)  host  and 
is  thus  constantly  bathed  by  the  digested  food 
which  the  latter  provides.  Relieved  of  the  necessity 
of  getting  its  own  food,  it  has  no  need  for  sense 
organs  or  for  apparatus  for  eating,  digesting,  or 
storing  food.     Digestive  system   and   sense  organs 


ORGANIC  RESPONSE 


287 


have  accordingly  disappeared  through  "  degenera- 
tion." The  fact  of  degeneration  is  a  famihar  accom- 
paniment of  the  habit  of  parasitism,  though  the 
method  by  which  it  comes  about  is  Httle  understood. 
As  in  the  malarial  parasite,  there  is  an  alternation 
of  hosts  in  the  life  cycle  of  the  tapeworm.  In 
most  cases  the  completion  of  this  cycle  requires 
that  one  host  shall  be  eaten  by  the  other.  It  follows 
therefore  that  such  parasites  are  most  numerous  in 
carnivorous  animals. 

Parasitism  in  Insects.  —  Another  group  in  which 
parasitism   flourishes  is   the   one  that   has   become 


Fig.  103.  —  Caterpillar  of  Sphinx  moth  with  cocoons  of  a  parasitic  wasp 
attached.  —  (Sanderson  and  Jackson.) 

adapted  to  all  possible  modes  of  existence  except 
that  under  the  sea,  —  the  insects.  A  few  insects 
are  parasitic  on  other  animals  {e.g.  the  bot-fly),  but 
the  majority  parasitize  other  insects.  A  very  large 
group  of  the  Hymenoptera  (ants,  bees,  and  wasps) 
are  parasitic,  and  many  of  them  specialize  on  ■  the 


288  GENERAL  BIOLOGY 

caterpillars  of  various  moths  and  butterflies.  Eggs 
are  laid  within  the  body  of  the  living  caterpillar, 
which  is  frequently  packed  full  of  the  resulting  larvse. 
These  go  through  a  rapid  development  and,  working 
their  way  through  the  outer  skin,  emerge  from  the 
caterpillar  and  spin  a  cocoon  on  the  outside.  Others 
wait  until  the  caterpillar  has  become  a  chrysalis, 
and  pupate  within  the  pupal  case  of  the  host.  There 
emerge  in  due  time,  not  a  butterfly,  but  a  number  of 
parasitic  wasps.  Not  infrequently  the  parasites 
themselves  are  parasitized,  and  even  these  secondary 
parasites  by  tertiary  ones.  In  some  cases  it  has 
been  discovered  that  the  original  parasitic  wasp  may 
lay  but  one  egg  in  the  larval  host,  but  that  this  egg 
fragments  into  scores  of  others  (polyembryony ) , 
each  of  which  develops  a  perfect  parasite.  In  these 
forms  the  parasitic  condition  is  passed  during  the 
developmental  stages,  and  the  mature  wasp,  using 
all  its  senses,  shows  no  trace  of  degeneracy.  There 
is  nothing  degenerative  about  parasitism  per  se,  but 
the  condition  is  merely  a  form  of  "  adaptation,"  an 
economy  of  nature  resulting"  in  simplification  of 
structure  through  the  loss  of  useless  parts. 

Sacculina.  —  An  extreme  case  of  the  degeneration 
incident  to  parasitism  is  found  in  a  crustacean, 
{Sacculina),  parasitic  upon  several  species  of  crabs. 
In  its  mature  condition  this  creature  has  the  form  of 
a  bag,  which  is  found  attached  to  the  abdomen 
of  its  host,  the  crab.  The  bag  is  full  of  eggs  which 
hatch  into  typical  crustacean  larvse.     It  has   been 


ORGANIC   RESPONSE 


289 


found  that  these  swim  about  until  they  encounter  a 
crab,  to  which  they  attach  themselves  at  the  root 
of  a  hair.  The  little  larva  then  forces  an  entrance 
into  the  body  of  its  host  and  begins  to  grow,  as  a 
plant  forces  its  roots  into  the  ground.     At  the  same 


Fig.  104.  —  Sacculina  attached  to  the  abdomen  of  a  crab:  ks,  the 
sac-like  parasite  giving  off  root-like  processes  that  permeate  the  body  of 
the  host.  —  (From  Lang,  after  Delage.) 


time  it  casts  off  bodily  the  abdomen  with  its  attached 
appendages,  and  its  larval  sense-organs,  including 
those  of  sight,  begin  to  retrogress  and  soon  disap- 
pear.    There  is  no  need  for  digestive  organs,  as  the 


290  GENERAL  BIOLOGY 

parasite  draws  its  sustenance  from  the  tissues  of  the 
crab  by  the  penetrating,  root-hke  ingrowth  men- 
tioned. The  latter  eventually  pervades  every  part 
of  the  crab's  body,  and  of  course  soon  causes  its  death, 
but  not  before  the  bag-like  Sacculina  has  matured 
its  eggs,  and  thus  insured  the  possibility  of  another 
generation. 

Association  among  Plants.  —  Nearly  all  the  de- 
grees of  association  that  exist  among  animals  are 
also  paralleled  in  the  plant  world,  modified,  of  course, 
by  the  very  different  conditions  that  obtain  among 
plants.  Plants  of  the  same  species  are  often  found 
associated  together,  but  this  is  usually  the  result  of 
accident  or  of  similar  favorable  conditions,  and  is, 
of  course,  not  comparable  to  the  gregariousness  of 
many  animals.  On  the  other  hand,  plants  of  dif- 
ferent species  are  sometimes  associated  together  in  a 
true  symbiosis.  One  of  the  most  remarkable  of 
these  is  the  association  between  the  roots  of  legu- 
minous plants  and  the  nitrogen-fixing  bacteria, 
which  has  already  been  described  in  a  previous 
chapter.  (See  p.  76.)  A  somewhat  similar  sym- 
biosis is  found  between  the  roots  of  trees,  such  as 
oak,  walnut,  apple,  maple,  etc.,  and  an  investing 
fungus.  The  latter  covers  the  root  as  a  mantle  or 
sheath  or  else  penetrates  the  cells  of  the  root  cortex. 
The  delicate  branches  (hyphse)  of  the  fungus  function 
as  root-hairs  for  the  host-plant. 

Lichens. — An  equally  remarkable  association  be- 
tween two  diverse  forms  of  plants  is  that  of  a  fungus 


ORGANIC   RESPONSE  291 

and  an  alga,  in  the  structure  known  as  lichens. 
Lichens  are  found  incrusting  rocks  and  tree-trunks 
and  are  frequently  beautifully  colored.  They  consist 
of  a  fungus  mycelium,  surrounding  various  uni- 
cellular or  filamentous  algse.  The  alga,  by  virtue 
of  its  chlorophyll,  synthesizes  carbohydrates,  which 


Fig.  105.  —  The  building  up  of  a  lichen  (Physcia  paraiina)  out  of  the 
alga  and  fungus;  A,  germinating  ascospore  (sp)  ;  the  filaments  of  the 
fungus  have  seized  upon  two  cells  (a)  of  Cystococcus  humicola ;  B,  more 
advanced  stage ;  sp,  ascospores  which  have  produced  a  web  of  filaments 
(hyphae),  enveloping  the  algal  cells  in  every  direction.  Magnified  about 
400  times.  —  (From  Scott,  after  Bonnier.) 

the  fungus  appropriates.  On  this  account  the 
condition  is  sometimes  referred  to  as  helotism, 
since. the  relation  is  that  of  master  and  slave. 

Parasitism  in  Plants.  —  As  in  animals,  plants 
are,   in    some    cases,   parasitic    upon    other    plants. 


292  GENERAL  BIOLOGY 

Especially  is  this  true  of  the  fungi.  Fungus  spores  of 
various  sorts  fall  upon  leaves  or  stems  of  a  green 
plant,  and  germinating,  gain  access  to  the  interior 
through  stomata  or  injuries  in  the  bark  or  by  directly 


Fig.  106.  —  A,  European  dodder  twining  about  a  hop  stem.  All 
but  the  uppermost  coils  show  the  groups  of  wartlike  swellings  from 
which  haustoria  penetrate  the  host  stem.  Natural  size.  B,  germination 
of  same.  The  various  stages  are  arranged  in  order  from  right  to  left. 
In  the  last  stage  the  seedling  has  found  a  suitable  support  and  has 
absorbed  all  the  reserve  food  in  the  thickened  lower  end,  which  has 
withered  and  died.  Magnified  about  two  diameters.  —  (From  Barnes, 
after  Kerner.) 

dissolving  the  outer  tissue,  and,  once  gaining  a 
foothold,  grow  and  ramify  in  all  directions,  producing 
blight,  rot,  or  abnormal  growths  (tumors) .  Familiar 
examples  are  the  "  rust  "  and  "  smut  "  of  wheat 
and  corn. 


ORGANIC  RESPONSE  293 

Higher  plants  in  several  instances  have  adopted 
parasitic  habits  and  have  thereby  become  profoundly 
modified,  usually  in  a  "  degenerative  "  way.  The 
dodder  is  a  climbing  plant  related  to  the  morning 
glory.  Its  seeds  germinate  in  the  usual  way,  but 
soon  the  seedling  attaches  itself  to  another  plant, 
and  casting  off  its  attachment  to  the  ground,  feeds 
thereafter  on  the  juices  of  its  host,  which  it  sucks 
up  through  organs  called  haustoria,  that  grow  fast 
to  the  leaves  and  stems  of  the  host-plant.  The 
mistletoe  is  another  (semi-)  parasitic  seed  plant  that 
grows  upon  the  oak.  Unlike  the  dodder  it  has  no 
ground-roots,  the  seeds  being  carried  by  birds 
from  tree  to  tree,  where  they  catch  in  crevices  in  the 
bark,  and  sprout. 

Association  of  Plants  and  Animals.  —  Nearly  all 
the  grades  of  association  that  have  been  described 
for  plants  or  for  animals  are  also  found  to  exist 
between  plants  and  animals. 

Many  crabs  are  protected  by  growths  of  seaweeds 
which  cover  their  carapace.  When  these  are  re- 
moved, the  crabs  plant  others.  Although  the  algse 
are  passive  members  of  this  partnership,  yet,  of 
course,  they  do  not  suffer  by  the  association. 

In  other  cases,  certain  algse  exist  in  a  combination 
with  simple  forms  of  animal  life  that  is  quite  analo- 
gous to  the  fungus-alga  relation  just  described  for 
the  lichens.  The  yellow  variety  of  Hydra  owes  its 
color  to  the  presence  within  its  endodermal  cells 
of  a  symbiotic  alga.     Ciliate  Protozoa,  sponges,  and 


294  GENERAL   BIOLOGY 

even  some  of  the  smaller  worms,  also  contain  similar 
algae,  usually  of  a  green  color.  These  are  called 
Zoochlorella,  and  they  function  by  synthesizing 
carbohydrates  for  their  animal  hosts. 

Of  course,  plants  suffer  from  the  attacks  of  her- 
bivorous animals,  and  not  infrequently  such  animals, 
if  small,  live  on  the  plant  upon  which  they  feed 
(the  aphids,  for  example).  To  call  this  parasitism 
would  be,  however,  a  rather  forced  use  of  the  word. 
On  the  other  hand,  animals  are  subject  to  the  at- 
tacks of  multitudes  of  plant-parasites.  These  be- 
long to  the  great  group  of  the  Fungi,  and  most  con- 
spicuous among  them  are  the  bacteria.  By  no 
means  all  of  the  bacteria  that  live  on  or  in  the  animal 
body  are  disease-producing,  yet  the  great  majority  of 
diseases  to  which  man  and  the  other  mammals  are 
subject  are  produced  by  bacteria  that  find  entrance 
to  the  body  and  multiply  there. 

Grafts.  —  A  condition  which  may  be  called  artificial 
symbiosis  occurs  in  grafting.  The  graft  or  scion 
is  a  twig  or  bud  or  some  other  portion  of  a  plant  which 
is  inserted  into  the  stem  of  another  (related)  plant, 
the  stock,  with  which  it  enters  into  an  intimate 
physical  relation,  and  the  two  behave  thereafter  as 
one  plant.  The  individual  characteristics  of  both 
elements  are  usually  preserved.  Thus  a  pear 
graft  on  a  quince  stock  produces  only  pears,  and 
different  sorts  of  fruits  may  be  grafted  upon  the  same 
stock.  An  application  of  this  principle  saved  the 
Bordeaux   vineyards  from  extermination  not  many 


ORGANIC  RESPONSE 


295 


years  ago.  The  phylloxera,  a  relative  of  the  corn- 
root  aphis  already  described,  infests  the  roots  of 
grape-vines.  The  introduction  of  this  pest  into 
French  vineyards  threatened  their  rapid  destruction 
until  it  was  discovered  that  the  wild  varieties  of 
American  grapes  are  immune  to  the  insect.  Ameri- 
can stocks  were  planted,  upon  which  were  grafted 
the  French  varieties  of  grape, 
thus  defeating  the  phylloxera, 
while  at  the  same  time  pre- 
serving the  peculiar  qualities 
of  the  French  fruit. 

Grafting  is  also  possible 
between  animals.  Tadpoles, 
moth-pupae,  worms,  etc.,  may 
be  cut  in  pieces  and  fastened 
together  in  all  sorts  of  ways 
without  destroying  individual 
life  and  growth.  Even  in 
mammals,  various  organs 
have  been  transplanted  from 
one  animal  to  another  (of  the  same  species)  with- 
out loss  of  function.  A  practical  application  of  the 
same  principle  is  used  by  surgeons  in  starting  a 
healing-process  in  large  burns  on  the  human  body 
by  the  grafting  on  of  bits  of  skin  taken  from  another 
person. 


Fig.  107.  —  A  common 
method  of  grafting:  A,  inser- 
tion of  two  scions  into  cleft  of 
stock  at  cambium  (growing) 
region ;  B,  wound  protected 
with  wax  to  prevent  drying 
out  of  tissues.  —  (From  Curtis, 
after  Bailey.) 


CHAPTER  IX 

SPECIES  AND  THEIR  ORIGIN 

Meaning  of  Species.  —  Throughout  our  discussion 
of  the  various  phases  of  organic  phenomena  we  have 
been  compelled  to  use  the  word  "  species  "  without 
defining  it,  although  its  meaning  must  have  been 
more  or  less  evident  from  the  context.  Indeed, 
there  is  no  term  in  general  use  in  Biology,  the  mean- 
ing of  which  is  so  vague  or  so  variously  interpreted, 
and  the  definition  of  which  is  so  difficult.  The 
Latin  word  "  species,"  which  has  been  directly 
incorporated  into  English,  means,  primarily,  "  form  " 
or  "  appearance,"  the  visible  structure  by  which 
anything  may  be  recognized ;  hence,  by  inference, 
the  word  came  to  mean  "  sort  "  or  "  kind." 

Until  the  early  decades  of  the  eighteenth  century, 
practically  all  serious  scientific  writing  was  in  Latin, 
and  the  word  "  species,"  when  used  in  describing 
different  kinds  of  animals  or  plants,  had  no  technical 
connotation.  The  early  naturalist,  in  naming  an 
unusual  form,  proceeded  as  any  person  would  in 
describing  a  friend;  that  is,  he  summarized  its  salient 
characteristics  in  a  brief  sentence.  The  lion  was  the 
"  Cat  with  a  tuft  at  the  end  of  the  tail."  ^     The 

^  "  Felis  Cauda  in  floccum  definente."  M.  J.  Brisson,  "  Regnum  Ani- 
male:  Quadrupedum,"  p.  194.     1756. 

296 


SPECIES  AND   THEIR  ORIGIN  297 

tiger  was  the  "  Yellow  cat,  variegated  with  long 
black  stripes."  ^  The  black  (water)  oak  was  the 
"  Maryland  oak  with  leaves  three-lobed  like  the 
sassafras."  ^ 

It  is  obvious  that  this  was  a  very  clumsy  and 
unsystematic  method,  if  the  forms  to  be  described 
were  at  all  numerous.  Particularly  was  it  impossible 
to  be  certain  whether  or  not  two  writers  were  dis- 
cussing the  same  thing.  As  new  forms  of  animals 
and  plants  were  discovered  the  confusion  became 
worse.  Finally,  in  the  middle  of  the  eighteenth 
century,  order  was  brought  out  of  the  chaos  by 
Linnaeus.  This  great  Swedish  naturalist  worked 
out  and  published  a  classification  of  all  known 
forms  of  animal  and  plant  life,  —  the  famous  Sys- 
tema  Naturce.  Not  only  was  each  kind  or  "  species  " 
described  in  a  brief  "  diagnosis,"  but  each  was 
given  a  double  name.  Thus,  all  the  members  of  the 
Cat  tribe  were  called  Felis,  and  the  group  was  called 
a  genus  (plural,  genera).  Each  kind  of  cat  was 
then  given  a  specific  name ;  thus,  the  house  cat  was 
called  Felis  domestica;  the  lion,  Felis  leo;  the  tiger, 
Felis  tigris;  the  black  oak,  Quercus  nigra,  etc.,  the 
technical  designation  of  the  form  being  compounded 
of  a  generic  and  a  specific  name.  This  binomial 
nomenclature  has  been  used  ever  since  Linnaeus' 
time,  and  affords  an  elastic  and  easily  intelligible 
system.     Not  only  are  closely  similar  species  grouped 

^  "  Felis  flava,  maculis  longis  nigris,  variegata."     L.c.  p.  195. 
2 "  Quercus  Marilandica,  folio  trifido    ad    sassafras   accidente.'*     M. 
Catesby,  "  Natural  History  of  Carolina,"  I,  p.  19.     1731. 


298  GENERAL  BIOLOGY 

together  in  a  genus,  but  similar  genera  are  in  turn 
assembled  in  families,  and  families  in  orders.  The 
genera  and  other  larger  groups  were,  of  course, 
artificial     and    abstract     categories.     Not    so    the 


Fig.  108.  — Carolus  Linnaeus  (1707-1778). 

species,  in  Linnaeus'  mind.  He  says,  "  There  are 
as  many  species  as  the  infinite  Creator  produced 
in  diverse  form  in  the  beginning,  and  these  forms, 
according  to  the  recognized  laws  of  reproduction. 


SPECIES  AND  THEIR  ORIGIN 

have  produced  others  always  Hke  themselves."  ^ 
This  conception  of  the  fixity  and  permanence  of 
species  was  not  always  held  so  firmly  as  it  was  during 
the  eighteenth  and  the  first  half  of  the  nineteenth 
centuries.  Milton  and  the  type  of  mind  of  which 
he  was  the  spokesman  doubtless  had  much  to  do 
with  riveting  this  idea,  essentially  a  theological 
dogma,  upon  the  popular  mind. 

Any  branch  of  natural  science  has  its  beginning 
in  the  classification  and  assembling  of  data.  The 
number  and  diversity  of  forms  of  plant  and  animal 
life  have  proved  so  great  that  until  quite  recently 
the  energies  of  naturalists  were  almost  exclusively 
directed  toward  the  classifying  and  naming  of 
species.  This  has  been  done  largely  on  the  basis 
of  "  outward  form,"  and  the  methods  of  the  scientist 
are  not  different  in  kind  from  those  of  the  non- 
biologist.  If,  for  example,  the  latter  should  be 
given  a  basket  of  fishes  of  all  sorts,  he  would  have  little 
difficulty  in  picking  out  by  sight  the  various  kinds, 
and  he  probably  would  make  but  few  mistakes.  If 
the  basket  should  contain  exclusively  fresh-water 
fishes,  it  might  be  considerably  more  difficult  to 
sort  accurately  the  various  kinds.  One's  ability  to 
do  so  would  depend  a  good  deal  upon  his  quickness 
of  eye  in  appreciating  details  of  structure.  These 
details  are  the  same  ones  that  have  already  been 
denoted  "  characters  "  in  another  connection.     Con- 

1  "  Species  tot  sunt,  quot  diversas  formas,  ah  initio  produxit  infinitum , 
Ens,  qucE  jormoe  secundum  generationis  inditas  leges  produxere  plures, 
at  sibi  semper  similis." 


300  GENERAL  BIOLOGY 

sciously  or  unconsciously  we  arrange  the  individuals 
of  a  group  by  the  common  possession  or  lack  of 
possession  of  certain  visible  characters.  Such  a 
method  of  discriminating  species  has  been  called  the 
diagnostic  method.  It  is  obvious  that  the  individual 
judgment  must  play  a  large  part  in  deciding  the  im- 
portance or  constancy  of  such  characters.  A  species 
judged  by  such  a  method  cannot  have  any  wholly 
accurate  definition,  so  long  as  the  personal  equation 
enters  so  largely.^ 

When  we  trace  resemblances  between  human 
beings,  we  usually  adopt  a  similar  method,  that  is, 
we  catalogue  their  physical  characters  and  group 
the  individuals  by  their  common  possession  of  such 
characters.  Thus,  in  spite  of  individual  peculiarities, 
we  have  no  difficulty  in  distinguishing  a  Swede 
from  an  Italian,  nor  a  Chinese  from  both.  The 
first  two  resemble  one  another,  in  spite  of  their 
differences,  more  than  they  resemble  the  Oriental. 
In  the  same  way,  a  child  frequently  resembles  its 
immediate  parents,  more  rarely  a  grandparent,  and 
much  more  rarely  a  cousin,  an  uncle,  or  a  more 
remote  relative.     We  accept  it  as  a  matter  of  course 

1  In  the  middle  of  the  eighteenth  century  Buffon  proposed  a  criterion 
of  species  that  avoided  this  diflSculty.  He  held  fertility  in  crossing  to 
be  the  test  of  specific  identity.  If  two  forms  were  sterile  when  inter- 
bred, they  were  distinct  species ;  if  not,  they  were  varieties  of  one  species 
provided  the  hybrids  themselves  were  not  sterile.  This  essentially 
scientific  hypothesis,  which  for  clearness  and  workableness  has  much  to 
commend  it,  had  the  curious  result  of  emphasizing  the  concept  of  Special 
Creation,  since  if  it  were  uniformly  true,  it  is  hard  to  see  how  new  species 
could  ever  arise. 


SPECIES  AND  THEIR  ORIGIN  301 

that  close  relatives  on  the  whole  look  much  alike, 
or  at  least  resemble  one  another  much  more  than 
do  remote  kin.  It  is  for  the  same  reason,  in  a  larger 
way,  that  the  diverse  types  of  Europeans  are  never- 
theless more  alike  than  are  Europeans  and  Mon- 
golians. The  different  races  of  mankind  are  usually 
classed  as  one  species  for  reasons'  that  we  will  not 
enter  upon  here,  but  the  same  phenomenon  is  found 
in  all  lower  animals  and  plants.  Individuals  of 
common  descent  thus  constitute  groups  which  in 
very  many  cases  are  identical  with  those  segregated 
by  the  diagnostic  method,  on  the  basis  of  a  common 
possession  of  characters.  Here,  then,  we  have 
another  criterion  of  species,  community  of  descent. 
Species  may  be  not  only  forms  which  share  in  com- 
mon certain  physical  characters ;  by  the  same  token 
they  also  share  a  common  ancestry.     . 

Polymorphism.  —  The  importance  of  the  latter 
factor  becomes  evident  when  we  consider  the  phe- 
nomenon of  dimorphism  and  polymorphism.  Many 
species  of  insects  are  known  in  which  one  sex  occurs 
in  more  than  one  form.  Thus,  in  one  of  our  common 
American  butterflies,  Papilio  turnus,  the  "tiger 
swallowtail,"  the  male  is  brilliant  yellow  with 
vertical  stripes  on  the  fore  wings.  In  the  northern 
range  of  the  species  (Canada  and  the  North  Central 
States),  the  female  is  similar  to  the  male,  but  in  the 
Southern  States,  in  addition  to  the  yellow  females, 
occurs  also  a  black  form  without  the  stripes,  or  with 
faint  indications   of  them.     For   a  long  time  this 


302 


GENERAL  BIOLOGY 


form  was  considered  a  distinct  species,  on  a  **  diag- 
nostic "  basis,  and  was  given  the  name  "  glaucus'' 

When  it  was  found,  how- 
ever, that  eggs  laid  by 
yellow  females  devel- 
oped into  both  glaucus 
and  typical  turnus,  and, 
conversely,  that  eggs  laid 
by  glaucus  developed 
also  into  turnus,  it  be- 
came necessary  to  con- 
sider them  one  species. 
The  different  forms 
which  are  assumed  by 
those  animal  species  in 
which  a  marked  alter- 
nation of  generation  oc- 
curs, and  the  different 
types  or  castes  of  ants 
and  their  relatives,  are 
further  examples  of  mor- 
phological diversity  in 
the  same  racial  inheri- 
tance. 

In  many  cases  species 
are  not  so  sharply  set 
off  from  one  another  as 
in  the  above  example. 
Often  the  characters  are 
This  is  particularly  the 


Fig.  109.  —  Tiger  swallowtail 
butterfly,  showing  the  two  forms  of 
females  :  the  upper  one  is  the  typical 
''turnus,"  the  lower  one  .is  the  ''glau- 
cus" type.  The  contrast  is  made 
much  more  striking  by  the  coloration, 
which  is  yellow  and  black  in  the 
turnus  form  and  solid  black  in  the 
glaucus  form.  —  (From  "  Elements 
of  Biology,"  copyright,  1907,  by 
George  William  Hunter.  Permission 
of  the  American  Book  Co.,  pub- 
lishers.) 


based  on  measurements. 


case  with  birds  and  mammals.     In  such  forms  it 


SPECIES  AND  THEIR  ORIGIN 


303 


is  often  found  that  series  of  individuals  from 
different  zoogeographical  regions  will  differ  markedly 
in  such   characters.     It   then    becomes    a   question 


Fig.  110.  —  Polymorphism  in  ants  (Cryptocerus  varians)  :  a,  soldier; 
b,  same  in  profile ;  c,  head  of  same  from  above ;  d,  worker  ;  e,  female ; 
/.male.  —  (From  Wheeler's  "Ants,"  published  by  the  Columbia  Uni- 
versity Press.) 


whether  intermediate  forms  may  be  found  which 
will  insensibly  grade  both  ways  into  the  two 
types.     If  such  are  discovered,  then  we  consider  the 


304  GENERAL  BIOLOGY 

aggregate  to  be  one  species  and  the  divergent  types 
to  be  "  geographical  races,"  or  **  varieties."  The 
criterion  by  which  we  decide  whether  or  not  a  given 
aggregate  of  individuals  is  a  "  true  species,"  or  only 
a  variety,  is  that  of  lack  of  intergradation  of  one 
or  more  (not  necessarily  all)  characters.  But  it  will 
be  seen  that  the  increase  of  our  acquaintance  with 
a  certain  group  of  related  species  may  necessitate 
a  constant  revision  and  rearrangement  of  them. 
Let  us  consider  for  a  moment  an  imaginary  species 
of  bird,  extending,  let  us  say,  from  the  Atlantic  coast 
to  the  Rocky  Mountains,  representatives  of  which 
show  a  wide  range  of  measurements  in  the  bill. 
Suppose  that  the  eastern  representatives  have  a 
bill  averaging  two  centimeters  in  length,  whereas 
the  western  ones  have  a  bill  averaging  four  centi- 
meters, but  with  every  gradation  between  the  two 
extremes.  Assuming  that  this  is  the  only  significant 
difference  between  them,  we  should  consider  the  whole 
aggregate  to  be  one  species,  with  two  well-marked 
geographical  varieties.  But  if  some  one  were  able 
to  willfully  w^ipe  out  of  existence  the  individuals 
from  the  middle  districts  (as  the  passenger  pigeon 
has  been  wiped  out  within  the  memory  of  the  present 
generation),  then  a  later  naturalist,  who  was  ignorant 
of  that  fact,  would  be  justified  in  considering  the 
eastern  and  western  forms  distinct  species.  On  the 
other  hand,  an  earlier  student  who  might  have  re- 
tained specimens  of  the  exterminated  intermediate 
types,  would  certainly  continue  to  consider  them  all 
one  species.     In  a  way,  either  would  be  right,  for 


SPECIES   AND   THEIR  ORIGIN  305 

the  illustration  shows  that  the  concept  of  species 
is  an  abstract  one.  The  species  is  real  enough,  but 
the  criterion  of  the  species  exists  in  the  student's 
mind,  just  as  does  that  of  the  genus  or  the  higher 
groups.  Such  species,  frequently  called  Linncean 
specieSy  are  but  convenient  categories  for  classifying 
aggregates  of  plants  and  animals  which  closely 
resemble  one  another  and  are  of  (assumed)  common 
descent. 

Elementary  Species.  —  It  will  be  recalled  that 
there  is  considerable  evidence  for  the  belief  that 
discontinuous  variations  or  mutations  are  of  a  very 
different  sort  from  fortuitous  variations.  Their 
constancy,  their  abrupt  origin,  and  their  behavior 
in  heredity,  sharply  set  them  off  from  the  latter. 
De  Vries  called  his  mutants  "  elementary  species." 
He  believes  that  they  are  not  in  any  sense  the  product 
of  environmental  influence,  as  geographical  "  vari- 
eties "  may  be,  but  are  distinct  entities.  There  have 
been  found  over  two  hundred  such  species  of  the 
"  whitlow  grass,"  Draba  verna,  and  more  than  that 
of  the  hawthorn  {Cratcegus).  It  is  obvious  that  to 
separate  and  name  all  of  these  would  defeat  the 
purpose  of  classification,  which  is  to  reduce  chaos 
to  order,  and  complexity  to  simplicity,  so  the  Lin- 
nsean  species  will  probably  continue  to  be  used  as 
the  practical  units  of  the  classifier. 

The  "  pure  lines  "  or  genes  which  have  been  dis- 
covered in  both  plants  and  animals  are  probably  the 
real  units  of  organic  nature,  the  centers  of  stability 


306  GENERAL  BIOLOGY 

of  the  systems  we  call  specific  units.  In  the  forms 
which  reproduce  sexually  —  and  they  are,  of  course, 
the  majority  of  organisms  —  the  interweaving  and 
mingling  of  diverse  genes  in  each  generation  is  such 
as  to  render  almost  futile  the  hope  that  they  can 
ever  be  unravelled  and  described.  The  Linnsean 
species  or  phaenotype  is  like  a  tangled  and  knotted 
skein  of  yarn,  each  strand  of  which  maintains  its 
individuality,  while  commingling  in  closest  union 
with  the  rest. 

The  Origin  of  Species 

To  Linnaeus  and  his  followers,  the  origin  of  his 
species  presented  no  problem.  They  had  been 
created  as  such  in  the  beginning,  and  had  persisted 
until  the  present.  The  relatively  small  number  of 
species  known  to  him  made  this  seem  reasonable, 
although  it  is  likely  that,  had  he  been  familiar  with 
the  enormous  number  of  forms  now  known,  he  would 
not  have  been  impressed  otherwise  than  with  the 
additional  evidence  of  the  omnipotence  of  the  Creator. 
There  is  very  good  reason  for  believing,  however, 
that  the  doctrine  of  Special  Creation,  as  it  has  come 
to  be  called,  is  not  tenable.  Practically  all  modern 
biologists  believe  that  the  species  of  animals  and 
plants  now  on  the  earth  have  not  always  been  here 
in  their  present  form,  but  that  they  have  become 
transformed  from  other  preexisting  types  and  that 
the  constant  changefulness  that  characterizes  the 
life   of   the  individual   is   equally   characteristic   of 


SPECIES  AND  THEIR  ORIGIN  307 

the  race.  In  the  nature  of  things,  direct  evidence 
for  such  an  hypothesis  is  not  abundant,  but  the  in- 
ferential evidence  is  overwhelming,  and  one  can 
hardly  work  with  natural  objects  at  first  hand  with- 
out being  constantly  impressed  with  it.  Since 
life  does  not  exist  apart  from  living  organisms,  and 
since  organisms  are  always  found  in  the  aggregates 
we  call  species,  the  question  of  the  evolution  of 
life  is  wrapped  up  in  that  of  the  evolution  of  species, 
and  the  origin  of  new  species  is  its  central  problem. 

Evidence  for  the  Evolution  of  Species 
IN  THE  Past 

The  age  of  the  earth  is  very  great,  and  for  only  a 
fraction  of  its  life  has  it  been  habitable  for  living 
organisms.  Yet,  looking  back  from  the  present  day, 
an  enormous  vista  of  time  opens  up  during  which  we 
find  evidences  in  the  rocks  of  the  existence  of  animal 
and  plant  forms,  the  great  majority  of  which  are  now 
extinct.  Only  the  scattered  fragments  have  been 
preserved.^     When,  however,  we  piece  together  this 

1  "The  affinities  of  all  the  beings  of  the  same  class  have  sometimes 
been  represented  by  a  great  tree.  I  believe  this  simile  largely  speaks 
the  truth.  The  green  and  budding  twigs  may  represent  existing  species ; 
and  those  produced  during  former  years  may  represent  the  long  succes- 
sion of  extinct  species.  At  each  period  of  growth  all  the  growing  twigs 
have  tried  to  branch  out  on  all  sides,  and  to  overtop  and  kill  the  surround- 
ing twigs  and  branches,  in  the  same  manner  as  species  and  groups  of 
species  Jbave  at  all  times  overmastered  other  species  in  the  great  battle 
for  life.  The  limbs,  divided  into  great  branches,  and  these  into  lesser 
and  lesser  branches,  were  themselves  once,  when  the  tree  was  young, 
budding  twigs ;  and  this  connection  of  the  former  and  present  buds  by 


308  GENERAL  BIOLOGY 

record,  we  find  that  without  exception  it  tells  the 
story  of  one  type  replacing  another,  and  of  simple 
types  giving  place  to  more  and  more  complex. 
Patiently,  fragment  by  fragment,  the  records  of  the 
rocks  have  been  brought  together  until  we  now  have, 
for  example,  the  nearly  complete  genealogy  of  the 
horse,  dating  back  to  a  five-toed  ancestor,  not  much 
bigger  than  a  rabbit. 

History  of  the  Elephant.  —  A  very  good  example 
of  the  successive  changes  in  the  transformation  of 
one  type  into  another  is  to  be  found  in  the  elephant 
and  its  genetic  predecessors.  The  ancestor  of  the 
modern  elephant,  the  remains  of  which  have  been 

ramifying  branches  may  well  represent  the  classification  of  all  extinct 
and  living  species  in  groups  subordinate  to  groups.  Of  the  many  twigs 
which  flourished  when  the  tree  was  a  mere  bush,  only  two  or  three,  now 
grown  into  great  branches,  yet  survive  and  bear  the  other  branches, 
so  with  the  species  which  lived  during  long  past  geological  periods,  very 
few  have  left  living  and  modified  descendants.  From  the  first  growth 
of  the  tree,  many  a  limb  and  branch  has  decayed  and  dropped  off ;  and 
fallen  branches  of  various  sizes  may  represent  those  whole  orders, 
families,  and  genera  which  have  now  no  living  representatives,  and  which 
are  known  to  us  only  in  a  fossil  state.  As  we  here  and  there  see  a  thin, 
straggling  branch  springing  from  a  fork  low  down  on  the  tree,  and  which 
by  some  chance  has  been  favored  and  is  still  alive  on  its  summit,  so  we 
occasionally  see  an  animal  like  the  Ornithorhynchus  or  Lepidosiren, 
which  in  some  small  degree  connects  by  its  affinities  two  large  branches 
of  life,  and  which  has  apparently  been  saved  from  fatal  competition  by 
having  inhabited  a  protected  station.  As  buds  give  rise  by  growth  to 
fresh  buds,  and  these,  if  vigorous,  branch  out  and  overtop  on  all  sides 
many  a  feebler  branch,  so  by  generation  I  believe  it  has  been  with  the 
great  Tree  of  Life,  which  fills  with  its  dead  and  broken  branches  the  crust 
of  the  earth,  and  covers  the  surface  with  its  everbranching  and  beautiful 
ramifications."  —  Darwin,  "The  Origin  of  Species,"  Chapter  IV. 


310  GENERAL  BIOLOGY 

found  in  Egyptian  deserts,  appears  to  have  been  a 
pig-like  animal  about  as  big  as  a  cow,  with  a  very 
little,  if  any,  snout  or  "  trunk  "  (of  course  only  the 


V 

u3 

1 

i 

7 

3       c:J 

.•...- 

.-^      f 

4 

/ 

( 

5          rf 

4 

Fig.  112.  —  Profile  views  of  ancestors  of  the  elephant:  1,  the 
Indian  elephant  (modern) ;  2,  the  American  mastodon  (Pleistocene) ; 
3,  Tetrabelodon  (Miocene,  France)  ;  4,  Paleomastodon  (Eocene,  Egypt), 
5,  Meritherium  (Eocene,  Egypt).  —  (From  Lankester's  "Extinct  Ani- 
mals," after  Andrews.) 

skeleton  is  preserved).  Both  jaws  were  heavy,  with 
peculiar  molar  teeth  and  large  prominent  incisor 
or  tusk-teeth,  those  of  the  lower  jaw  nearly  as  long 
as  those  of  the  upper,  but  more  horizontal.     These 


SPECIES  AND   THEIR  ORIGIN  311 

remains,  which  have  been  named  Meritherium,  are 
found  in  Eocene  ^  formations. 

In  the  same  strata  are  found  also  fossils  of  another 
type  which  has  been  called  Paleomastodon.  In  this 
form,  we  find  the  same  sort  of  molar  teeth,  but  the 
tusks  of  the  upper  jaw  are  much  longer  than  those 
of  the  lower.  In  the  Miocene  era,  much  later  than 
Eocene,  we  find  the  Tetrabelodon,  which  is  charac- 
terized by  a  very  great  horizontal  extension  of  the 
long  upper  incisors,  now  to  be  called  tusks,  and  a 
corresponding  extension,  not  of  the  lower  incisors, 
but  of  the  lower  jaw  itself.  The  head  of  this  animal 
was  apparently  extended  in  a  long  bony  projection, 
composed  of  the  two  tusks  and  the  lower  jaw,  on 
which  doubtless  rested  an  extension  of  the  upper  lip 
in  the  form  of  a  snout  or  trunk.  Back  in  the  jaws,  we 
find  the  typical  molars.  In  America,  we  find  quanti- 
ties of  the  skeletal  remains  of  Mastodon,  a  huge 
elephant-like  creature  in  which  the  lower  jaw  has 
greatly  shortened,  the  lower  incisors  have  disappeared, 
and  the  upper  ones  (the  tusks)  are  enormously  en- 
larged, curving  up  and  inward.  Back  in  the  jaw 
we  find  the  same  type  of  molars,  reduced  in  number 
and  increased  in  size.  Finally,  in  the  modern 
elephant,  we  have  an  animal  with  a  greatly  "  fore- 
shortened "  skull,  in  the  lower  jaw  of  which  there  is 
only  room  for  one  single  ribbed  molar  tooth  at  a  time. 
The  upper  incisors  or  tusks  are  large,  and  curve 
downward,  and  between  them  is  the  long  sensitive 

^  The  Eocene  period,  the  earliest  of  the  Cenozoic  era,  dates  back 
probably  three  million  years. 


312 


GENERAL  BIOLOGY 


flexible  trunk  which  compensates  for  the  clumsiness 
of  the  rest  of  the  body. 

In  this  survey  it  is  not  implied  that  each  of  these 
forms  turned  one  into  another,  but  they  do  indicate 
the  very  probable  path  of  transformation  which 
the  elephant  has  followed  in  its  derivation  from  the 
more  usual  type  of  mammals. 


Fig.    113.  —  Appendix   vermiformis    of    kangaroo   (at   left);    of  human 
embryo  (at  right). —  (From  Jordan  and  Kellogg,  after  Wiedersheim.) 

Vestigial  Structures. — Another  sort  of  evidence 
for  the  transformation  of  species  is  to  be  found  in 
the  possession,  on  the  part  of  animals  now  living,  of 
useless  vestiges  that  correspond  with  similar  func- 
tional structures  possessed  by  more  primitive  types 
which  may  be  assumed  to  resemble  the  ancestors 
of  present-day  species.  The  most  striking  of  these 
structures  are  the  gill-clefts,  reminiscent  of  an  aquatic 
life  and  characteristic  of  fishes,  but  likewise  developed 
in  the  embryos  of  reptiles,  birds,  and  mammals, 
—  even  man  himself,  although  in  the  higher  verte- 


SPECIES  AND  THEIR  ORIGIN 


313 


brates  they  soon  close  over.  Numerous  other  such 
vestiges  might  be  cited.  The  vermiform  appendix 
is  a  now  useless  or  even  dangerous  legacy  from  some 
herbivorous  ancestor.  Another  useless  relic  of  the 
ancestral  past  is  the  muscular  equipmmt  of  the 
human  ear,  sometimes  so  com- 
plete (or  so  well  innervated)  that 
their  possessor  has  a  quite  un- 
human  capacity  for  moving  that 
organ.  Indeed,  some  one  hun- 
dred and  eighty  vestigial  organs 
have  been  recorded  in  man  (Wie- 
dersheim).  In  plants,  likewise, 
are  to  be  found  many  such  sur- 
vivals. Thus  in  the  Cycads, 
belonging  to  the  most  primitive 
group  of  the  seed-plants,  the 
sperm-cell  is  provided  with  cilia 
like  those  of  an  alga,  which  are, 
however,  useless,  since  zygosis 
does  not  take  place  in  the  water. 
Vestigial  structures,  such  as  have 
been  just  described,  were  likened 
by  Darwin  "  to  the  unsounded  letters  in  many 
words,  such  as  the  *  o '  in  leopard,  the  *  b '  in 
doubt,  and  the  *  g '  in  reign,  which  are  quite 
functionless,  but  tell  us  something  of  the  past  his- 
tory of  the  word."  ^ 

The  Origin  of  Species. — Granting  that  new  species 
have  come  or  are  now  coming  into  existence,  by  some 

1  Quoted  from  Thomson  and  Geddes'  "Evolution." 


Fig.  114.  — Two  cil- 
iated sperms  of  Cycas 
revoluta  just  previous  to 
their  breaking  away  from 
each  other  to  swim  in  the 
watery  secretion  of  the 
pollen  tube.  —  (After 
Miyake.) 


314  GENERAL  BIOLOGY 

form  of  natural  transmutation,  we  may  ask  what 
explanation  has  science  to  offer  of  the  method  of 
their  appearance  ?  Speaking  generally,  two  different 
answers  have  been  offered  to  this  question :  That  of 
Lamarck,  and  that  of  Darwin.^ 

Darwinism.  —  The  Darwinian  theory  of  the  origin 
of  species  rests  upon  three  generalizations.  First, 
in  every  species  of  animal  and  plant,  even  the  very 
slowest  breeding  one,  there  is  produced  an  enor- 
mously greater  number  of  individuals  than  can 
possibly  find  food  or  foothold.  Some  of  these,  of 
course,  survive  as  the  persistent  species,  the  rest 
perish.^  This  is  the  famous  "  struggle  for  existence," 
from  which  few,  if  any,  individuals  are  exempt. 
Secondly,  the  fact  of  variation,  which  has  already 
been  discussed,  calls  to  mind  that  in  this  horde  of 
individuals  every  possible  sort  of  variation  will  be 
found.  Some  of  these  will  be  favorable  to  survival, 
others  a  handicap ;  obviously,  when  competition  is 
so  fierce,  the  chances  are  strong  that  the  individuals 
which  possess  the  unfavorable  variations  should  go 
down  before  those  endowed  with  the  favorable  ones. 
For  example,  a  slight  difference  in  the  speed  of  an 

1  Some  would  add  also  De  Vries'  theory  of  Mutation,  mentioned  in 
a  previous  chapter. 

2  An  ordinary  mosquito  hatching  from  the  egg  reaches  maturity  and 
lays  her  own  eggs  ten  days  afterward.  A  single  female  lays  about  four 
hundred  eggs,  half  of  which  become  females.  If  a  single  female  should 
hatch  on  April  first  and  lay  her  quota  of  eggs  ten  days  later,  on  July  1, 
ninety  days  later,  if  all  lived  the  progeny  would  number  102,914,592,- 
864,480,008,004,001  mosquitoes  ! 


SPECIES  AND  THEIR  ORIGIN  315 

animal  may  determine  which  of  two  will  be  killed 
and  eaten  and  which  escape;  a  slight  difference  in 
resistance  to  low  temperature  will  determine  which 
of  two  plants  will  be  killed  by  frost  and  which  sur- 
vive. This  universal  phenomenon  was  termed  by 
Darwin,  "  The  Survival  of  the  Fittest."  It  may  be 
better  called,  perhaps,  the  survival  of  the  best  adapted, 
since  the  criterion  of  survival,  or  of  "  Natural  Selec- 
tion," as  Darwin  called  it,  is  the  degree  to  which  the 
organism  is  adapted  to  its  environment.  Since  "  like 
tends  to  produce  like,"  Darwin  held  that  the  individ- 
uals that  have  survived  on  account  of  their  favor- 
able variations  will  tend  to  reproduce  individuals 
of  the  same  type.  But  the  inorganic  environment  is 
no  more  stable  than  organic  nature.  Excluding 
the  titanic  changes  which.  Geology  teaches  us, 
have  been  going  on  for  long  periods  of  time,  the 
minor  changes  of  climate  and  physical  conditions 
are  constantly  nullifying  the  delicate  adjustments 
that  have  come  into  existence  through  the  natural 
selection  just  described.  New  criteria  for  the  selec- 
tion of  the  survivors  in  the  struggle  will  become 
operative,  and  in  consequence  a  different  type  will  be 
preserved.  It  is  not  even  necessary  for  the  environ- 
mental changes  to  become  profound.  As  we  have 
seen,  in  connection  with  fortuitous  variation,  free 
interbreeding  tends  to  maintain  a  single  mode,  but 
if  a  group  of  variants  should  be  segregated  and 
prevented  from  mutual  intercrossing,  no  other 
factor  than  chance  need  be  called  upon  to  bring 
about  a  divergence  of  two  modes.     Darwin's  atten- 


316  GENERAL  BIOLOGY 

tion  was  attracted  to  the  problem  in  the  first  place 
by  observing  that  in  the  various  islands  of  the 
Galapagos  group,  off  the  western  coast  of  South 
America,  although  numbers  of  genera  of  land  animals 
are  to  be  found  on  each  of  the  islands,  as  well  as  on 
the  mainland,  yet  each  island  has  its  own  species, 
differing  slightly,  but  definitely,  from  those  of  ad- 
jacent islands.  There  is  little  doubt  but  that  at 
one  time  the  islands  were  all  connected  with  one 
another  and  with  the  mainland,  and  populated  with 
one  species  of  rabbit  or  of  grasshoppers  or  other 
forms.  When  the  islands  came  into  existence  through 
the  depression  of  the  land,  the  consequent  segrega- 
tion and  isolation  of  the  different  groups,  and  their 
enforced  inbreeding,  called  into  being  the  new  and 
divergent  types  now  to  be  found  there. 

Such  a  segregation  need  not  even  be  physical.  A 
mutual  infertility  may  arise  between  groups  of  a 
species  in  a  common  habitat  which  would  just  as 
effectively  segregate  them,  so  far  as  reproducing 
the  species  is  concerned,  as  if  a  physical  barrier  were 
erected. 

Darwin's  hypothesis  of  Natural  Selection  is  thus  a 
theory  of  the  origin  of  species  (evolution  being 
assumed)  through  the  mutual  relation  of  organisms 
to  their  environment,  such  that  the  unadapted  are 
eliminated  and  a  changing  environment  produces 
changing  types.  Its  most  important  feature  is  the 
emphasis  which  it  lays  upon  the  passivity  of  the 
organism  itself.  The  selection  is  purely  mechanical, 
and  the  conclusions  as  regards  the  first  two  premises 


SPECIES  AND  THEIR  ORIGIN  317/ 

(the  enormous  overproduction  of  individuals,  and 
consequent  elimination)  are  incontrovertible. 

Lamarck's  Theory.  —  We  have  seen,  in  a  previous 
chapter,  that  one  of  the  fundamental  qualities  of 
living  matter  is  that  of  response,  and  that  environ- 
mental stimuli  frequently  call  forth  advantageous 
reactions  (such  as  the  callusing  of  the  hand  through 
friction).  That  the  reaction  may  also  be  disad- 
vantageous is,  of  course,  equally  true.  Moreover, 
the  fact  that  the  organism  is,  as  a  rule,  exquisitely 
adapted  to  the  particular  environment  in  which 
it  is  found  is  one  of  the  most  conspicuous  facts  of 
nature.  Lamarck  contended  that  the  continued 
effect  of  such  response  made  its  impression  on  the 
inheritance  of  the  organism,  or,  to  use  a  technical 
phrase,  that  such  "  acquired  characters  "  are  in- 
herited. Use  and  disuse  thus  play  a  large  part  in 
Lamarck's  theory.  From  another  standpoint,  the 
will  of  the  organism  came  into  play,  since  he  held 
that  the  need  for  the  development  of  an  organ  is  a 
causative  factor  in  its  production.  Swimming  birds 
acquired  their  web  feet  by  spreading  their  toes  to 
avoid  sinking  in  the  mud ;  the  giraffe,  its  long  neck 
by  the  inherited  effect  of  generations  of  stretching 
after  the  leaves  of  trees.  Of  course  plants  are 
equally  as  well  adapted  to  their  environment  as  are 
animals,  but  any  question  of  use  and  disuse,  or 
particularly  of  such  a  psychical  factor  as  need,  must 
be  ruled  out  in  the  case  of  plants.  Lamarck,  in  this 
case,  laid  especial  emphasis  upon  such  factors  as 
light,  heat,  moisture,  etc. 


318  GENERAL  BIOLOGY 

Critique  of  the  Darwinian  Theory.  —  Darwin's 
work,  "  The  Origin  of  Species,"  was  published  in 
1859,  and  the  entire  first  edition  was  sold  on  the  day 
of  issue.  This  was  evidence  of  the  acute  public 
interest  in  the  subject  at  that  time.  The  publica- 
tion was  merely  the  spark  in  the  powder  magazine, 
for  the  idea  of  organic  evolution  had  been  in  the  air 
for  a  century  and  a  half,  and  had  been  steadily 
gaining  strength.  The  following  ten  or  fifteen  years 
were  occupied  with  controversy  and  heated  polemic, 
for  the  vital  argument  was  not  so  much  the  hypothe- 
sis of  Natural  Selection  as  the  theory  of  Organic 
Evolution  versus  the  doctrine  of  Special  Creation. 
As  to  the  conclusion  of  the  issue  there  could  be  no 
doubt,  and  when,  finally,  the  theory  of  Evolution 
was  definitely  established,  that  of  Natural  Selection 
was  accepted  along  with  it,  although  of  course  the 
former  did  not  necessarily  involve  the  latter.^ 

The  chief  result  of  Darwin's  great  generaliza- 
tion was  an  extraordinary  development  of  natural 
science  and  the  extension  of  the  field  of  biological 
inquiry  in  every  direction.  Darwin  had  spent  his 
life  in  the  patient  accumulation  of  data  on  which 
he  based  his  generalization,  and  he  was  not  unaware 

^ "  History  warns  us,  that  it  is  the  customary  fate  of  new  truths  to 
begin  as  heresies  and  to  end  as  superstitions ;  and,  as  matters  now  stand, 
it  is  hardly  rash  to  anticipate  that,  in  another  twenty  years,  the  new 
generation,  educated  under  the  influences  of  the  present  day,  will  be 
in  danger  of  accepting  the  main  doctrines  of  the  "Origin  of  Species" 
with  as  little  reflection,  and  it  may  be  with  as  little  justification,  as  so 
many  of  our  contemporaries,  twenty  years  ago,  rejected  them."  — 
T.  H.  Huxley,  "  The  Coming  of  Age  of  the  '  Origin  of  Species.' "     1880. 


SPECIES  AND   THEIR  ORIGIN  319 

of  weak  links  in  his  own  argument  which  he  candidly 
avowed.  These  became  more  emphasized  as  time 
went  on,  and  a  host  of  investigators  continually 
added  to  the  facts'that  were  most  difficult  to  explain 
by  the  theory  of  Natural  wSelection.  We  have  space 
for  but  a  few  of  these  objections.  (1)  The  basis  of 
elimination  or  preservation  is  the  usefulness  of 
the  organ  whose  variations  serve  as  the  criterion 
for  selection,  but  there  are  thousands  of  very 
stable  characters  which  must  be  of  wholly  indifferent 
value  to  the  organism.  One  of  the  largest  groups 
of  the  ground-beetles  is  divided  into  two  sub-groups 
containing  hundreds  of  species  by  the  invariable 
distinction  of  the  possession  of  one  microscopic  hair 
above  the  eye  or  of  two.  (2)  Again,  while  it  may  be 
recognized  that  the  emphasis  on  a  certain  structure 
may  be,  so  to  speak,  of  selection  value,  yet  the 
minute  differences  of  fluctuating  variations  can  hardly 
count  one  way  or  the  other.  Thus  one  author  calls 
attention  to  the  polar  bear,  whose  white  coat  must 
be  of  great  utility  to  him  in  stealing  upon  his 
prey  unobserved.  Without  doubt,  this  species  has 
evolved  from  a  type  of  the  more  usual  coloration, 
but  "  did  the  fortuitous  appearance  in  his  coat  of  a 
spot  of  white  hairs  as  large  as  a  dollar  or  a  pancake 
give  some  ancient  brown  bear  such  an  advantage 
in  the  struggle  for  existence  as  to  make  him  or  her 
the  forerunner  of  a  new  and  better-adapted  sort  of 
bear?"  Darwin  recognized  this  difficulty,  but 
thought  that  the  struggle  for  existence  was  so  keen 
that  the  slightest  difference,  however  slight,  might 


320  GENERAL  BIOLOGY 

decide  between  survival  and  extermination.  (S)  Al- 
lowing for  the  fact  that  certain  small  variations  are 
advantageous  to  their  possessors,  and  granting  that 
of  the  hosts  of  individuals  born  into  existence, 
but  a  minute  fraction  can  hope  to  survive,  yet  in 
many  cases  chance  must  play  a  larger  part  in  their 
extermination  than  the  possession  of  any  kind  of 
morphological  or  physiological  character  whatever. 
(4)  Most  significant  of  all,  perhaps,  is  the  experimen- 
tal demonstration  that  artificial  (and  by  inference, 
natural)  selection  has  narrow  limits.  Beyond  a 
certain  point  (see  p.  219)  the  pull  of  the  mysterious 
factor  of  regression  prevents  any  further  progress 
in  that  direction. 

Critique  of  the  Lamarckian  Theory.  —  The  chief 
characteristic  of  man  as  distinguished  from  other 
animals  is  the  fact  that  he  "  looks  ahead  "  and  shapes 
means  to  his  own  ends.  It  is  difficult  to  avoid 
unintentionally  attributing  the  same  purposes  to 
the  abstraction  we  call  "Nature  "  or  the  "species." 
Animals  and  plants  react  to  many  stimuli.  Often 
these  reactions  are  advantageous  and  these  we  note ; 
frequently  they  are  quite  the  contrary,  and  these  we 
sometimes  fail  to  remember.  Man  stores  up  food 
to  provide  for  a  future  time  of  want.  When  a  potato 
stores  up  starch  in  the  tuber,  what  more  natural 
than  to  think  of  it  in  the  same  way,  —  the  plant  is 
anticipating  its  own  needs?  But  it  has  been  dis- 
covered that  the  formation  of  tubers  is  directly  due 
to  the  presence  of  an  infecting  fungus  and  does  not 
occur  in  its  absence. 


SPECIES  AND  THEIR  ORIGIN  321 

The  same  dangers  beset  the  path  of  the  unwary 
who  argue  from  the  Lamarckian  standpoint.  The 
need  for  a  useful  organ  is  evident,  but  we  are  by  no 
means  justified  in  assuming  that  the  "  need " 
brought  about  the  existence  of  the  organ.  Even 
man  cannot  "  by  taking  thought,  add  one  cubit  to  i 
his  stature."  The  Lamarckian  argument  rests  upon 
the  transmission,  in  heredity,  of  the  results  of  environ- 
mental influences  upon  the  soma ;  in  other  words,  the 
"inheritance  of  acquired  characters."  On  d  priori 
grounds,  Weismann  sought  to  show  that  this  was 
impossible,  since  the  germ-plasm  gives  rise  to  the 
soma  and  is  unaffected  by  its  accidents.  Moreover,  , 
the  greatest  variety  of  experiment  has  been  attempted 
for  many  years,  in  an  effort  to  secure  the  hereditary 
transmission  of  any  sort  of  such  acquired  characters, 
with  universally  negative  results.^  One  of  the 
most  elaborate  of  these  was  carried  out  by  a  Ger- 
man botanist  who  transplanted  some  2500  different 
kinds  of  mountain  plants  to  the  lowlands  and 
studied  them  for  several  years  in  comparison  with 
their  lowland  relatives.  He  found  that  the  alpine 
environment  had  made  no  permanent  change  in 
their  habit  or  structure. 

It  is  not  even  necessary  to  assume  such  a  dis- 
tinction as  that  between  germ-plasm  and  soma- 
plasm,  a  distinction  that  is  sometimes  difficult  to 
maintain,  in  order  to  appreciate  the  improbability 
of  the  inheritance  of  environmental  effects.  Ani- 
mals  and   plants   are   complexes   of   matter   whose 

1  With  one  very  doubtful  exception. 


322  GENERAL  BIOLOGY 

nature  is  ultimately  dependent  upon  some  sort  of 
molecular  organization  of  which  we  are  profoundly 
ignorant.  But  we  are  almost  equally  ignorant  of 
the  nature  of  the  organization  of  the  simplest 
chemical  compounds.  We  know  nothing,  for  in- 
stance, of  the  relation  existing  between  the  oxygen 
and  the  hydrogen  that  gives  water  its  peculiar 
properties.  These  properties  are  inherent,  and  were 
they  to  alter  fundamentally,  our  whole  body  of 
chemical  theory  would  be  upset.  On  the  other 
hand,  the  manner  in  which  these  properties  are 
revealed  to  us  is  determined  primarily  by  external 
(environmental)  conditions,  i.e.  those  of  tempera- 
ture and  pressure.  We  think  of  H2O  as  a  liquid 
because  that  is  the  form  that  our  usual  conditions 
of  temperature  and  pressure  cause  it  to  assume. 
But  the  liquid  state  is,  of  course,  no  more  charac- 
teristic of  the  compound  than  the  gaseous  or  solid 
state.  If  we  should  keep  a  quantity  of  water  at  a 
temperature  of  0°  C.  for  a  hundred  years,  we  should 
have  no  reason  for  supposing  that  its  liquid  nature 
would  be  altered  in  the  slightest  if  the  temperature 
were  finally  raised  ten  degrees.  Indeed,  we  may  say 
that  it  is  the  specific  characteristic  of  water  to  be  a 
solid,  a  liquid,  or  a  gas,  at  definite  calculable  levels 
of  temperature. 

In  the  same  way,  the  adjustment  of  internal 
relations  to  external  ones  is  a  specific  characteristic 
of  the  organism.  To  put  it  another  way,  the  solid 
condition  of  ice  is  not  "  caused  "  or  "  produced  " 
by  the  lowering  of  the  temperature,   except  in  a 


SPECIES  AND  THEIR  ORIGIN  323 

figurative  sense.  It  is  the  essential  nature  of  H2O 
to  be  a  solid  at  low  temperature.  In  the  same  way 
the  climatic  conditions  of  the  mountains  do  not 
cause  the  profound  .modifications  in  the  habit  of 
alpine  plants  in  contrast  to  their  congeners  of  the 
lowlands.  The  nature  of  the  species  is  to  respond, 
morphogenetically,  in  one  way  to  one  sort  of  environ- 
ment, and  in  another  way  to  another  sort  of  environ- 
ment without  being  intrinsically  altered  by  either. 
For  the  succession  of  individuals  is  a  continuous 
stream,  and  there  is  no  absolute  break  between  one 
generation  and  another.  In  other  words  the  species, 
like  the  organism,  has  a  unity  unaffected  by  its  sur- 
roundings. This  consideration  does  not  affect  the 
idea  that  species  do  alter  with  respect  to  their  in- 
trinsic nature,  and  that  in  the  course  of  time  new 
species  come  into  existence  from  preexisting  ones. 
It  emphasizes,  however,  the  significance  of  the 
internal  factors  involved  and  the  relative  unimpor- 
tance (in  a  direct  and  permanent  way)  of  the  action 
of  the  environment. 

In  conclusion  it  may  be  said  that  biologists  are  by 
no  means  so  positive  in  giving  their  allegiance  to  one 
theory  or  another  of  the  origin  of  species  as  they 
might  have  been  a  generation  ago.  The  concept 
of  Evolution,  that  is,  of  the  progressive  changeful- 
ness  of  organic  Nature  and  the  descent  of  present- 
day  species  by  modification  of  preexisting  types, 
forms  the  basis  of  all  modern  biological  work. 
As  to  the  method  of  the  evolutionary  process  there 
are  several  opinions,  and  it  may  very  well  be  that 


324  GENERAL   BIOLOGY 

each  is  but  a  part  of  a  true  explanation,  the  complete 
key  to  which  will  be  discovered  only  by  subsequent 
researches.  The  painstaking  and  brilliant  work  of 
Darwin  can  never  be  set  aside  in  spite  of  the  fact  that 
we  may  be  compelled  to  doubt  the  universality  of 
his  theory  of  Natural  Selection.  Instead  of  hig 
fluctuating  variations,  however,  it  may  be  that  the 
basis  for  selection  is  something  like  Johanssen's 
"  genes,"  or  those  vague  units  of  organization  that 
reveal  themselves  to  us  as  Mendelian  unit-char- 
acters. But  we  must  have  a  far  deeper  insight  into 
the  physics  and  chemistry  of  the  organism  than 
is  now  available  before  we  can  begin  to  formulate 
an  hypothesis  as  to  the  real  nature  of  these  units. 


INDEX 


The  numbers  refer  to  pages  ;  illustrations  are  indicated  by  boldface  type. 


Abiogenesis,  130. 

Acetylene,  49. 

Adaptation,  general,  259  ;  aquatic, 
260 ;  aerial,  262 ;  subterranean, 
263;  protective,  266;  for  seed 
dispersal,  278. 

Adaptive  response,  253. 

Adrenalin,  88. 

Aerial  adaptation,  262. 

Aerobic  organisms,  63. 

Agamy,  167. 

Alimentary  system,  in  animals, 
104;   in  plants,  126. 

Alisma,  reproduction,  180. 

Alternation  of  generations,  ani- 
mals, 171 ;    plants,  175. 

Amitosis,  93. 

Amoeba  angulata,  32 ;  A.  proteus, 
31 ;   reproduction,  166. 

Anabolism,  56. 

Anaerobic  organisms,  63. 

Anaesthetics,  67. 

Analogy,  47. 

Anisogamy,  144. 

Annelid,  structure,  119. 

Anolis,  254. 

Anosia  plexippus,  271. 

Ant-guests,  265. 

Antiseptics,  67. 

Anti-toxins,  256. 

Aphis,  life-history,  167;  with  ants, 
281. 

Apogamy,  183. 

Appendix  vermiformis,  312. 

Aquatic  adaptation,  260. 

Assimilation,  30. 

Associations,  of  animals,  279;  of 
plants  and  animals,  293. 


Bacteria,    fission,    136 ;     nitrogen- 
fixing,   76 ;    putrefactive,   74. 


Beans,  pure  lines  in,  221. 

Beroe,  261. 

Biffin,  experiments  with  wheat,  235. 

Binomial  nomenclature,  279. 

Biogenesis  and  abiogenesis,   130. 

Blastogenic  variations,  213. 

Blastopore,  162. 

Blastula,  160. 

Bodo  lens,  142. 

Bonnet,  191. 

Bose,  R.  C,  248. 

Bougainvillea,  172. 

Budding,     136;      in    Hydra,     138; 

in  Syllis,  139 ;    permanent,  139 ; 

in  animals,  132. 

Carbohydrates,  12. 

Carbon  cycle,  72. 

Care  of  the  young,  272. 

Carnivorous  plants,  59. 

Cell,  18,  20  ;   various  kinds,  22. 

Cellular  structure  of  leaf,  19. 

Cellulose,  41. 

Cell  wall,  an  adaptive  structure,  24. 

Centrosome,  21,  94;   nature  of,  98. 

Choetopterus,  169. 

Chemical  agents,  effect  of,  on 
growth,  103. 

Chemical  environment,  245. 

Chromatin,  23. 

Chromosome,  94. 

Chrysanthemum  segetum,  variation, 
207. 

Ciona  intestinalis,  40. 

Circulatory  system,  in  animals, 
115.  116;    in  plants,  126. 

Cleavage,  of  animal  egg,  157. 

Clover  as  a  fertilizer,  76. 

Colloids,  15. 

Coloration,  protective,  267;  ag- 
gressive, 269. 


325 


326 


INDEX 


Combustion,  49 ;    and  respiration, 

64. 
Commensalism,  280. 
Conducting  organs,  39. 
Conjugation,  cytoplasmic,  151 ;    in 

animals,      155 ;       nuclear,      152 ; 

partial,     151 ;      in    Paramecium, 

165  ;   in  Protozoa,  164. 
Connective  tissues,  41. 
Conservation  of  energy,  50. 
Cork,  function  of,  124. 
Corn-root  louse,  282. 
Correlation,  209. 
Creation,  special,  299,  306. 
Ctenodrilus,  132. 
Ctenophor,  261. 

Curve,  variation,  202  ;   skew,  204. 
Cycads,  ciliated  sperms,  313. 
Cycle  of  the  elements,  68. 
Cytoplasm,  20. 

Darwin  quoted,  307-308. 

Darwinism,  314  ;   critique  of,  318. 

Death,  4. 

Defects,  Inheritance  of,  239. 

Denitrification  of  the  soil,  75. 

Determiner,  194. 

De  Vrics,  Mutation  theory,  208. 

Differentiation,    in     animals,     104 ; 

in  plants,  121. 
Digestion,    29;     specialization    in, 

42 ;    in  higher  animals,  62. 
Dioncea,  59. 

Disease,  inheritance  of,  236. 
Dissimilation,  56. 
Division  of  labor,  36. 
Dodder,  292. 

Draba  verna,  species  of,  305. 
Droserd,  59. 
Drosophila,  234. 

Ectoderm,  162. 
Egg  and  sperm,  150. 
Electric  ray,  78. 
Electric  response,  247. 
Electricity  in  organisms,  79. 
Elements,  cycle  of,  in  Nature,  68. 
Elephant,  history  of  the,  310. 
Endoderm,  162. 
Endoskeleton,  110,  111. 


Energesis,  66. 

Energid,  25. 

Energy,  conservation  of,  50. 

Enterokinase,  85. 

Environment,  242,  245. 

Enzymes,  82. 

Epigenesis,  192. 

Eudorina,  147. 

Eugenics,  240. 

Evolution,  of  plants,  184 ;  of  species, 

307. 
Excretory    organs,     117,    118;     in 

Protozoa,  119. 
Exoskeleton,  112. 

Fats,      production      of,      in      the 

organism,  55. 
Ferments,  84. 
Ferns,  reproduction,  176. 
Fertilizers,  71. 
Filial  regression,  219. 
Firefly,  81-82. 
Fission,  in   Metazoa,  132 ;  in  lower 

plants,  135. 
Flagellata,  33. 
Flame  cells,  119. 
Flowers,  178. 
Flying  fish,  263. 
Foods,  60  ;    fate  of,  in  animals,  61 ; 

influence  of,  on  response,  258. 
Fortuitous  variation,  202. 
Fowl's  combs,  inheritance,  231. 

Galton,  Francis  241 ;  ancestral 
inheritance,  217 ;  device  illus- 
trating variation,  201. 

Galvanotropism,  251. 

Gamete,  141. 

Ganglia,  107,  108. 

Gastrulation,  161. 

"Genes,"  324. 

Germ  layers,  162. 

Germplasm,  150. 

Glands,  44 ;   ductless,  86. 

Glycogen,  12. 

Grafting,  295. 

Growth,  90. 

Haemophilia,  239. 
Hawthorn,  species,  305. 


INDEX 


327 


Heat  of  organisms,  79, 

Heredity,  214;  racial,  216;  Gal- 
ton's  law  of,  217;  selection  in, 
220. 

Hermit-crab,  266. 

Heteromorphosis,  190. 

Hologamy,  141. 

Holozoic  and  holophytic  nutrition, 
58. 

Homology,  46. 

Hookworm,  286. 

Hormone,  89. 

Horse,  evolution  of,  309. 

Huxley,  T.  H.  (quoted),  318. 

Hydra,  43;  budding,  138;  re- 
generation, 186. 

Hydroid,  colonial,  17?, 

Hydrophytes,  275. 

Hypertrophy,  compensatory,  253. 

Immunity,  255. 

Index  of  variability,  203. 

Ingestion,  29. 

Inheritance,  215;  ancestral,  217; 
Mendeliau,  223 ;  sex-limited, 
233  ;  of  disease,  236 ;  of  defects, 
239. 

Insects,  parasitism  in,  287. 

Ions,  10;   ion-proteins,  11. 

Irritability,  30,  244. 

Isogamy,  142. 

Johanssen,  221. 

Karyogamy,  142,  152. 
Katabolism,  57. 
Katydid,  268. 
Kidney,  121. 

Lamarck's  theory  of  evolution,  259, 

317 ;    critique,  320. 
Lantern  fish,  81. 
Leaf,  cellular  structure,  19. 
Leucocytes  of  frog,  27. 
Lichens,  291. 
Life  and  death,  4. 
LigKt,   effect  of,   on   growth,   100 ; 

on  response,   245;    actinic  rays, 

101 ;  in  animals,  80. 
Linnaeus,  298. 


Linophryne  lucifer,  81. 
Lipas,  83. 

Liverwort,  reproduction,  175. 
Living  and  non-living,  2. 
Locomotor  organs,  32. 
Loeb,   J.,   experiments,    170;    with 
Ciona,  40. 

Mean,  203. 

Malarial  parasite,  284. 

Mastodon,  311. 

Mastigamoeba,  33. 

Maturation,  153. 

Mechanical  tissue  in  plants,  124. 

Mechanism  and  vitalism,  194. 

Megagamete,  148 ;  spore,  ger- 
mination, 179. 

Mendelism,  223  ff. 

Meristic  variation,  205. 

Mesophytes,  277. 

Metabolism,  57. 

Metamerism,  135. 

Metaplasm,  21. 

Microgamete,  148. 

Microspore,  germination,  179. 

Milk,  as  an  emulsion,  13. 

Mimicry,  269. 

Mitosis,  92,  95,  97;  abnormal,  88, 
97. 

Mode,  203. 

Monarch  butterfly,  270. 

Morphogenesis,  185 ;  theories  of, 
190. 

Morphogenetic  response,  256. 

Mosses,  reproduction  in,  175. 

Mougeotia,  146. 

Movement,  77  ;   of  plants,  123. 

Muscle  cells,  38. 

Muscles  in  insects,  114. 

Muscular  system,  113. 

Mutations,  207. 

Myrianida,  fission,  133. 

Nasturtiums,  orientation,  249. 
Natural  Selection,  315. 
Nematoda,  286. 
Neo- vitalism,  196. 
Nephridium,  120. 

Nervous  system  of  a  caterpillar, 
108. 


328 


INDEX 


Nettles,  inheritance  in,  228. 
Nitrogen-fixing  bacteria,  76. 
Nitrogen  loss  through  plant  growth, 

71. 
Noctiluca,  79,  80. 
Normal  curve,  202. 
Notochord,  112. 
Nucleo-plasma  relation,  99. 
Nucleus,  20. 
Nutrient  solutions,  70. 

(Enothera,  mutations,  208. 

Ontogenesis,  129. 

Organic  response,  242. 

Organic  synthesis,  52. 

Organisms,      destruction     of,     72 ; 

putrefactive,  73. 
Organs,  sensory,  107. 
Origin  of  species,  306. 
Oxidation,  48  ;   chemistry  of,  65. 
Oxygen,  role  of,  in  metabolism,  62. 
Ozone,  49. 

Palcemon,  heteromorphosis,  190. 

Papilio  turnus,  302. 

Paramecium,  34;    electric  response, 

251, ;    conjugation  in,  165 ;    pure 

lines  in,  223. . 
Parasitic  and  saprophytic  nutrition, 

58. 
Parasitism,     in      Protozoa,       283 ; 

in  worms,  285  ;    in.  insects,  287  ; 

in  plants,  292. 
Parthenogenesis,     167;     in'  wasps, 

168;      artificial,     169,     170;      in 

in  plants,   183. 
Peas,  Mendel's  experiments,  225. 
Pelagic  adaptation,  261. 
Pelagonemertes,  261. 
Pepsin,  83. 
Phagocytes,  29. 

Phosphorescent  animals,  79-82. 
Photophores,  264. 
Photosynthesis,  53. 
Phylloxera,  295. 
Pipa,  272. 
Plants,    association    among,    290 ; 

parasitism  in,  292;    response  in, 

250  ;     sexual   reproduction,    174  ; 

evolution  of,  184 ;  fission  in,  135. 


Plastogamy,  151. 
Poisons,  66. 
Pollen-tube,  179. 
Polyembryony,  189. 
Polymorphism  in  species,  301 ;    in 

ants,  303. 
Preformation,  191. 
Proteins,     10 ;     production    of,    in 

the  organism,  55. 
Protista,  26. 
Protoplasm,  chemistry,  8  ;   physics, 

of,  14 ;    organization  of,   17. 
Protozoa,  26  ;    parasitism  in,  284  ; 

movements  of,   77 ;    conjugation 

in,  164. 
Pseudopodia,  28. 
Pure  lines,  221. 
Putrefactive  organisms,  74. 

Reaction,  243. 

Reduction,  153. 

Regression,  filial,  219. 

Regulation,    185;     in   Hydra,   186; 

Stentor,    187 ;      sea-urchin    blas- 

tulae,  188. 
Reproduction  as  a  growth  process, 

131 ;   in  plants,  174 ;   sexual,  141. 
Respiration,  64. 
Response,  organic,  242  ;    nature  of, 

246  ;    electric,  243  ;    unsymmetri- 

cal,    248 ;     adaptive,    253 ;     mor- 

phogenetic,  256. 
Reversion,  232. 

Rhinoceros  beetles,  variation,  205 
Rust  in»wheat,  292. 

Sacculina,  289. 

Salts,  inorganic,  importance  of, 
10. 

Sea-urchin  larvae,  regeneration, 
188. 

Secretin,  89. 

Secretion,  41 ;  internal  and  ex- 
ternal, 86. 

Seeds,  stored  foods  in,  128 ;  dis- 
persal, 278. 

Seed  plants,  reproduction  in,  177, 
180. 

Segmentation,  metameric,  135. 

Selection,  Natural,  316. 


INDEX 


329 


Sensory  organs,  107. 

Sex-limited  inheritance,  233. 

Sexual  differentiation,  147 ;  re- 
production, 141 ;    in  plants,  174. 

Sigma,  in  variation,  203. 

Skeletal  structures,  of  animals, 
110;    in  plants,  123. 

Skew  curves,  204. 

"Sleep"  in  plants,  122. 

Soma-plasm,  150. 

Sparrow,  variation,  211. 

Special  creation,  299. 

Specialization,  in  conducting  organs, 
39 ;  in  digestion,  42 ;  in  loco- 
motor organs,  32  ;  and  generali- 
zation, 45. 

Species,  Linnaean,  305 ;  criteria  of, 
300  ff. ;  elementary,  305  ;  origin 
of,  306,  313 ;  meaning  of  term, 
296. 

Specific  energy,  law  of,  247. 

Spermatocyte,  154. 

Spiral  valve,  106. 

Spirogyra,  146. 

Spore  formation,  140. 

Sporophyte,  175. 

Starch,  synthesis  of,  in  the  leaf,  53. 

Stature,  variation  in,  200. 

Stentor,  regeneration  in,  187. 

Stephanosphcera,  144. 

Stimulus,  243. 

Stomias  boa,  264. 

Struggle  for  existence,  129. 

Stylonychia,  35. 

Substantive  variation,  205. 

Suckers,  137. 

Sundew,  59. 

Surinam  toad,  273. 

Survival  of  the  fittest,  315. 

Suspended  animation,  7. 

Syncytium,  91. 

Synthesis  in  the  organism,  52. 

Synthetic  products,  artificial,  3. 

Syllis  ramosa,  139. 

Tardigrades,  6. 
Teleology,  195. 


Temperature,  effect  of,  on  response, 

244;   on  growth,  101. 
Thyroiodin,  88. 
Tiger  butterfly,  302. 
Tonus,  246. 
Torpedo,  78. 
Trichosphoerium,  151. 
Trochosphoera,  261. 
Tunicate,  40. 
Typhlosole,  106. 

Unit  characters,  223. 

Use  and  disuse  in  evolution,  317. 

Variation,  198;  causes  of,  212; 
correlated,  209 ;  discontinuous, 
205  ff. ;  effect  of  conditions  upon, 
211;  Galton's  device,  201;  in 
human  stature,  200;  in  beech 
leaves,  202;  types  of  curves, 
202 ;   substantive,  205. 

Vegetative  reproduction,  133. 

Venus'  flytrap,  59,  122. 

Vermiform  appendix,  312. 

Vertebrate  and  invertebrate,  109. 

Vestigial  structures,  312. 

Vitalism  and  mechanism,  194. 

Volvox  globator,  149. 

Vorticella,  37. 

Waller's  criterion  of  life,  7. 
Water,  in  protoplasm,  9. 
Weismannism,  193. 
Wheat,  inheritance  in,  235. 
Wolff,  K.  F.,  192. 
Worms,  parasitic,  288. 

Xerophytes,  276. 
Xylotrupes,  variation  in,  205. 

Yeast,  growth  of,  131. 
Yolk,  159. 

Zaitha,  272. 

Zoochlorella,  294. 

Zoospore,  174. 

Zygosis,  145 ;   in  animals,  155. 

Zymogen,  85. 


:2^ 


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' 

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